Java Card CAP files are permitted to contain new, or custom, components. All new components not defined as part of this specification must not affect the semantics of the specified components, and Java Card virtual machines must be able to accept CAP files that do not contain new components. Java Card virtual machine implementations are required to silently ignore components they do not recognize.

New components are identified in two ways: they are assigned both an ISO 7816-5 AID ( Section 4.2, AID-based Naming) and a tag value. Valid tag values are between 128 and 255, inclusive. Both of these identifiers are recorded in the custom_component item of the Directory Component ( Section 6.4, Directory Component).

The new component must conform to the general component format defined in this chapter, with a tag value, a size value indicating the number of bytes in the component (excluding the tag and size items), and an info item containing the content of the new component.

A new component file is stored in a JAR file, following the same restrictions as those specified in Section 4.1.3, JAR File Container. That is, the file containing the new component must be located in the /javacard subdirectory of the JAR file and must have the extension .cap.