Most Digital switches have functions that internally maps ISUP to INAP messages.
Messages like IAM (Initial_Address_Messge), ACM (Address_Complete_Message), CPG (Call_Progress_Message), REL (Release), etc.. are a part of ISUP. INAP – Intelligent Network Application Part – has it’s own messages like: InitialDP(), Connect(), RequestReportBCSM, etc..
Good starting point is to look at the ETSI/3GPP standards for CS-1, CS-2 and CAP V2.
For IN, CS-1 (Capability Set 1) is the foundation.