I have mentioned before that UICC is the physical card and 2G SIM/USIM/ISIM are applications on the UICC card. The IMS SIM holds data provided by the IMS Operator, generally the same operator that would provide USIM services that would allow to camp on the 3G or LTE network.
Private User Identity: This identifies the user uniquely with the IMS operator and is used when the user registers with the IMS network. This is used by the operator to check the subscription and which services the user can avail of.
Public User Identity: A user can have multiple public identities that can be used for different services. To avail a particular service, user has to register with the particular public identity that has been allowed for that service.
Security Keys: Security keys are used for authentication to the IMS Network.
Home Network Domain Name: This is the name of the entry point that the user uses to register. This makes sure that a users request is sent to the Home Network.
Access Rule Reference: This is used to store information about which personal identification number needs verification for accessing a particular application
Address of P-CSCF: If it is not possible do dynamically find the Proxy-Call Session Control Function then this address is helpful
Administrative Data: Some of this could be operator specific proprietary information