An ASN is simply defined by letting the Reqd attribute of a field to be “Seq” and having the corresponding Entity-Set or Relationships with fields created (or recreated). To ensure proper handling of the existing data, it’s strongly recommended to use DC to perform this operation.
The next step is to set the initial value for this ASN like in these examples:
SET SEQUENCENUMBER MyEnt 1
SET SEQUENCENUMBER AnotherEnt 1000
SET SEQUENCENUMBER Customers 5000001
See the documentation of the command SET SEQUENCENUMBER for details.