Modules
Here is a list of all modules:

Asynchronous Messaging | Structure for managing asynchronous state of CoAP resources
Block Transfer | API functions for handling PDUs using CoAP BLOCK options
Clock Handling | Default implementation of internal clock
DTLS Support | API functions for interfacing with DTLS libraries
DTLS Support (Internal) | Internal API functions for interfacing with DTLS libraries
Event API | API functions for event delivery from lower-layer library functions
Logging Support | API functions for logging support
Option Filters | API functions for access option filters
Pseudo Random Numbers | API functions for gerating pseudo random numbers
Rate Control | The transmission parameters for CoAP rate control ("Congestion Control" in stream-oriented protocols) are defined in https://tools.ietf.org/html/rfc7252#section-4.8
Resource observation | API functions for interfacing with the observe handling (RFC7641)
Sessions (Internal) | Structures, Enums and Functions that are not exposed to applications
String handling support | API functions for handling strings
Subscribe (Internal) | Structures, Enums and Functions that are not exposed to applications
URI Parsing Functions | CoAP PDUs contain normalized URIs with their path and query split into multiple segments