Evidence of the qualifying employment abroad, a description of the specialized knowledge and how it will be used, and, where the blanket route is used, credentials supporting the professional element. The employer supplies the blanket approval itself and confirmation that both entities are covered by it.
Check the entity list against the actual transfer
Groups reorganise, and the entity employing the person abroad or receiving them is not always the one named in the approval. Verify both against the list before building the file. Where an entity is missing, that is a structural question for counsel rather than a paperwork oversight, and it may decide the route on its own without any reference to the employee's merits.
Add one verification that takes an afternoon and occasionally decides the matter: confirm which legal entity actually employs the person abroad and which will receive them, using payroll records and the employment contract rather than the organisation chart. In groups that reorganise frequently, the entity on someone's payslip is not always the one people name in conversation, and an intra-group transfer that everyone treats as internal may cross a boundary that matters. Check both names against the approval's entity list before anything else is prepared.
Hypothetical example: a mass spectrometry calibration specialist has been paid by a service subsidiary since a reorganisation two years earlier, while everyone including the specialist describes their employer by the group's trading name, and the payslip settles a question that no amount of description would have resolved.