The evidence should let a reviewer reconstruct the classification without guessing. Keep the legal entity name, establishment and service countries, business service description, Annex row, customer-facing service, size-cap data, special-case analysis, and final classification.
For size, keep the latest approved accounting-period data, annual work units, turnover excluding VAT and other indirect taxes, balance-sheet total, ownership and voting relationships, proportional partner data, and 100% of linked-enterprise data unless already consolidated. Under the Recommendation, crossing or falling below a ceiling normally changes SME status only when it occurs over two consecutive accounting periods, so keep both periods and document any case-specific calculation issue.
Article 3(4) guidance points to practical list data: entity name, address, contact details, email addresses, IP ranges, telephone numbers, relevant sector and subsector, and Member States where in-scope services are provided. Use that as an evidence model, then follow the applicable national mechanism.