Scope 3 for suppliers sounds like a project measured in months, because the GHG Protocol defines fifteen categories. In practice, a supplier is rarely asked for all fifteen. Customer questionnaires and bank templates ask for two of them first.
Scope 3 for suppliers: the two categories
Business travel (category 6) covers employees travelling for work in vehicles you do not own: flights, trains, buses and cars. Employee commuting (category 7) covers the journeys between home and work.
Both can be built from records you already have. Travel comes from agency statements, expense claims and booking confirmations. Commuting comes from a short staff survey or HR data.
How to log them
For travel, enter each trip with its mode, and for flights the haul (short, medium or long) and the class. A monthly total per mode is enough. For commuting, record for each facility how many employees travel by each mode, the average one-way distance and the working days, once a year or when headcount changes. Survey-based figures are marked as estimated, which is fine, because they are estimates. The step-by-step version is in Scope 3 business travel and commuting.
What to say about the other thirteen
If a questionnaire asks for purchased goods, transport, waste or any other category outside your inventory, say that it is outside the current boundary. A stated boundary is accepted. A number you invented is not, and it puts every other number on the form in doubt.
Karbon360 reports its own boundary the same way: two of fifteen categories, on every report, with the other categories on the roadmap by customer demand. The full list, with examples of what each one contains, is in Scope 3 categories explained.
When a larger Scope 3 is justified
Some customers will eventually want purchased goods or transport as well, particularly for manufacturers whose raw materials are the largest part of their footprint. Build it when a customer asks for it by name, not before. Scope 1 and Scope 2, done properly and traced, are the foundation every larger inventory sits on.