Number sequences are used to generate readable, unique identifiers for master data records and transaction records that require identifiers. A master data record or transaction record that requires an identifier is referred to as a reference.
Before you can create new records for a reference, you must set up a number sequence and associate it with the reference
See this Microdoft user guideline :
- Number sequences overview : https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/fin-ops/organization-administration/number-sequence-overview
- Set up number sequences on an individual basis : https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/fin-ops/organization-administration/tasks/set-up-number-sequences-individual-basis?toc=/dynamics365/retail/toc.json
- Set up number sequences using a wizard : https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/fin-ops/organization-administration/tasks/set-up-number-sequences-wizard?toc=/dynamics365/retail/toc.json
AX / Dynamics 365 For Finance and Operations blog 
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