Purchased goods and services (category 1) almost always come first and account for 40 to 70% of scope 3. The rest depends on the sector: use of sold products (category 11) exceeds 80% of a carmaker's scope 3, investments (category 15) make up most of banks' and insurers' footprints, downstream transport contributes heavily in e-commerce and distribution, and medicines contribute massively to a healthcare establishment's footprint. In most companies, 3 or 4 categories out of 15 explain 80% of the total, and these are where physical data should be collected first.
Read the source article: Scope 3: definition, 15 categories and calculation method