Because there is no document with detailed specifications of the migration tool,
partners are not able to estimate the budget for upgrade project from AX2009 to D365fO
The partners need to know actual tables/features/modules which data can be migrated from AX2009 to D365fO perfectly.
https://docs.microsoft.com/en-us/dynamics365/unified-operations/dev-itpro/migration-upgrade/data-migration-tool