Defined by the Trusted Computing Group(TCG), each abstraction layer adds onto the previous one and adds new software functionalities
FAPI - Functional API
ESAPI - Enhanced SAPI
SAPI - System API Interface
Standardized API for compatibility across platforms, also defined by TCG
On top of the TCTI typically stands SAPI
TPM Interface Specification, the low-level interface used between a device and the TPM.
The lowest possible abstraction between a programmer and any Trusted Platform Module