The IBM HSM can support one of two different cryptographic APIs, and you can reload your HSM firmware at any time to switch from one to the other.
The first is CCA, the IBM Common Cryptographic Architecture. IBM has offered CCA since the introduction of its first HSM products in 1989, and it is used by many systems. CCA provides a general-purpose set of cryptographic functions, but its main strength is in support of finance industry payments applications. The second is EP11, or Enterprise PKCS#11. This is a newer addition to the IBM HSM, but it provides the de-facto industry standard PKCS#11 API which is used by a wide variety of software written by many companies. EP11 offers a wide variety of general-purpose, secure-key only cryptographic functions.
As implied above, your choice between CCA and EP11 may be dictated by the applications you plan to use with the HSM.
If your application is for payment systems or other related banking operations, you should choose CCA.
If your application is written to use the industry-standard PKCS#11 API functions, you should choose EP11.
If neither of the conditions above apply to you, either CCA or EP11 may meet your requirements if the necessary cryptographic functions are supported. While both support most common cryptographic functions, there are differences and you can see what each one supports on the individual pages for CCA and EP11.
Your choice may also be driven by preferences or experience in your organization. For example, if your programmers have used one of the two APIs before, they may prefer to go in that same direction for a new application.