Customer Interface(RACUST) is a concurrent program that is responsible for the import and update of basic AR-related customer information from open interface tables to AR Customer tables. An import program can be used to format and load date from feeder systems to the open interface tables.
The TCA API is an integrated set of code designed in a highly modular fashion which can be used to insert and update entities in the TCA model with data obtained from various systems. Because of their modular design, TCA APIs offer more easier and specific access to the TCA entities compared to the Customer Interface.
The choice of which one to use ultimately depends on the user and their specific requirements. The following documents might help in making the choice:
Metalink Note#201242.1: TCA Customer Interface & API White Paper
Metalink Note#269121.1: Technical Uses of Customer Interface and TCA-API
Metalink Note#201243.1: Using TCA API’s Including Examples
Oracle Receivables User Guide(for documentation on Customer Interface)
Oracle Trading Community Architecture API User Notes
Sample codes from Metalink for using both Customer Interface(RACUST) and TCA APIs under various scenarios are attached here.
Notes on Oracle Applications 