You've seen the system API functions, but the question that hasn't been answered yet is how command byte streams are transmitted to the TPM and how the application receives response byte streams from the TPM. The answer is the TPM Command Transmission Interface (TCTI). You saw this briefly in the description of the Tss2_Sys_Initialize call. This call takes a TCTI context structure as one of its inputs. Now we will describe this layer of the stack in detail.
The TCTI context structure tells the SAPI functions how to communicate with the TPM. This structure contains function pointers for the two most important TCTI functions, transmit and receive, as well as less frequently used functions such as cancel, setLocality, and some others described shortly. If an application needs to talk to more than one TPM, it creates multiple TCTI contexts and sets each with the proper function pointers for communicating with each TPM.
The TCTI context structure is a per-process, per-TPM structure that is set up by initialization code. It can be set up at compile time or dynamically when the OS is booted. Some process has to either discover the presence of TPMs (typically a local TPM) or have a priori knowledge of remote TPMs and initialize a TCTI context structure with the proper function pointers for communication. This initialization and discovery process is out of scope of the SAPI and TCTI specification.
The most frequently used and required function pointers, transmit and receive, do what you'd expect them to. Both of them get a pointer to a buffer and a size parameter. The SAPI functions call them when they're ready to send and receive data, and the functions do the right thing.
The cancel function pointer supports a new capability in TPM 2.0: the ability to cancel a TPM command after it's been transmitted to the TPM. This allows a long-running TPM command to be cancelled. For example, key generation can take up to 90 seconds on some TPMs. If a sleep operation is initiated by the OS, this command allows early cancellation of long-running commands so that the system can be quiesced. 
The getPollHandles function pointer comes into play when SAPI is using the asynchronous method of sending and receiving responses—that is, the Tss2_Sys_ExecuteAsync and Tss2_Sys_ExecuteFinish functions. This is an OS-specific function that returns the handles that can be used to poll for response-ready conditions.
The last function pointer, finalize, is used to clean up before a TCTI connection is terminated. Actions that are required upon connection termination, if any, are performed by this function.
TCTI can be used at any level in the TPM stack where marshalled byte streams are being transmitted and received. Currently, the thinking is that this occurs at two places: between the SAPI and the TAB, and between the RM and the driver.
-  The cancel capability is specified in the TCG PC Client Platform TPM Profile (PTP) Specification. TPMs that support other platforms may not include the cancel command.