Setting the Scope 3 boundary means deciding which activities and which emissions to include in the inventory. Unlike Scope 1 and 2 — where the boundary is determined by the organisational consolidation approach — the Scope 3 boundary involves additional judgements about relevance, materiality, and the limits of reasonable data collection.
The Two-Step Boundary Decision
Setting the Scope 3 boundary involves two sequential decisions:
Step 1: Which of the 15 categories are relevant? This was covered in lesson 0.4. Companies screen all 15 categories and identify those that are significant based on the size of estimated emissions and the expectations of key stakeholders.
Step 2: Within each relevant category, which specific activities to include? Each category can span many individual activities. The company must decide where to draw the line — which suppliers, which product lines, which transport legs, which geographies to include.
The standard requires companies to include all significant Scope 3 emissions and to justify any exclusions. The boundary is not meant to be drawn to minimise reported emissions — it should reflect a complete and honest picture of the value chain's climate impact.
The Significance Test
An activity within a Scope 3 category is significant if it contributes meaningfully to total Scope 3 emissions. The standard does not define a numerical threshold for significance, but offers practical guidance:
- Activities that represent more than 1–5% of total Scope 3 emissions for a category are generally significant
- Activities where the data quality could be substantially improved may be prioritised even if currently small
- Activities that are growing rapidly may be significant in future even if currently minor
In practice, the significance test is applied through the screening assessment — estimating all activities using spend-based or average-data methods, then ranking by estimated magnitude.
The Primary Activities Concept
For many categories, the standard recommends including the primary activities of direct value chain partners, but not extending indefinitely upstream. For example:
- Category 1 (Purchased Goods): Include the cradle-to-gate emissions of direct suppliers (Tier 1). Companies may also include Tier 2 and beyond if data is available and those tiers are significant — but are not required to trace every component back to raw earth.
- Category 4 (Upstream Transport): Include transport legs the company directly purchases from third-party logistics providers. Do not need to include the upstream manufacturing of the truck itself (that would be counted in the truck manufacturer's Category 11).
Think of the boundary as deciding how many links of a chain to count. The GHG Protocol says: count all the links you know are significant; stop extending the chain when the additional links become negligible. A food company must count its direct dairy suppliers, should probably count the farms those dairy processors source from, but does not need to count the emissions from manufacturing the farm machinery used on those farms.
Mandatory vs. Optional Activities
The standard distinguishes between activities that shall be included and those that are optional:
Mandatory inclusions:
- All significant categories (as identified through screening)
- All activities within a significant category that contribute meaningfully
- Direct operational emissions at the boundary of the Scope 3 activity (e.g., combustion at a third-party facility processing the company's goods)
Optional inclusions:
- Scope 3 emissions of suppliers and customers (their own value chains)
- Activities beyond Tier 1 in the supply chain unless material
- Emissions from very small or highly fragmented sources where data collection is impractical
Documenting the Boundary
The standard requires companies to document:
- All categories evaluated — including those determined to be not relevant, with justification
- Excluded activities within relevant categories — with explanation of why they are not significant or why data collection is not feasible
- Methodologies applied — which calculation method was used for each category or sub-activity
This documentation serves as the audit trail for internal review, external assurance, and year-on-year consistency tracking.
Boundary Changes Over Time
Once a company has set its Scope 3 boundary and base year, it should apply the consistency principle — using the same boundary year over year. However, as data improves, companies should expand their boundary to include previously excluded activities that turn out to be more significant than initially estimated. Any boundary expansion that materially changes the inventory requires base year recalculation.
The standard stops short of requiring companies to include their suppliers' own Scope 3 emissions in Category 1. A company buys steel from a steelmaker. The steel's cradle-to-gate emissions (Category 1 for the buyer) include the steelmaker's Scope 1, Scope 2, and upstream Category 1 emissions (like iron ore mining). In principle, the buyer could extend further — accounting for the mine's own Scope 3. The standard permits but does not require this. In practice, most companies stop at the Tier 1 supplier's cradle-to-gate boundary, acknowledging that extending the chain further introduces rapidly increasing uncertainty without commensurately better decision-making.
Key Takeaways
- 1Setting the Scope 3 boundary involves two decisions: which of the 15 categories are relevant, and which specific activities within each category to include
- 2The standard requires inclusion of all significant emissions and justification for any exclusions - the boundary must not be drawn to minimise reported numbers
- 3Activities contributing more than 1-5% of a category's total emissions are generally considered significant
- 4Document all boundary decisions thoroughly - which categories were evaluated, what was excluded and why, and which methods were applied
- 5Boundary changes that materially affect the inventory require base year recalculation to maintain comparability over time