Modules
Here is a list of all modules:

ASN.1 Support (Internal) | CoAP ASN.1 Structures, Enums and Functions that are not exposed to applications
Application I/O Handling | API functions for Application Input / Output
Application I/O Handling (Internal) | Internal API functions for Application Input / Output
Asynchronous Messaging | API functions for Async "separate" messages
Asynchronous Messaging (Internal) | CoAP Async Structures, Enums and Functions that are not exposed to applications
Block (Internal) | Structures, Enums and Functions that are not exposed to applications
Block Transfer | API functions for handling PDUs using CoAP BLOCK options
Cache Support | API functions for CoAP Caching
Cache Support (Internal) | CoAP Cache Structures, Enums and Functions that are not exposed to applications
Clock Handling | Default implementation of internal clock
Context Handling | API functions for handling PDUs using CoAP Contexts
Context Handling (Internal) | CoAP Context Structures, Enums and Functions that are not exposed to applications
DTLS Support | API functions for interfacing with DTLS libraries
DTLS Support (Internal) | CoAP DTLS Structures, Enums and Functions that are not exposed to applications
Encode / Decode API | API functions for endoding/decoding CoAP options
Event API | API functions for event delivery from lower-layer library functions
Logging Support | API functions for logging support
Observe Subscription (Internal) | CoAP Observe Subscription Structures, Enums and Functions that are not exposed to applications
Option Filters | API functions for access option filters
PDU | API functions for PDUs
PDU (Internal) | CoAP PDU Structures, Enums and Functions that are not exposed to applications
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 Configuraton | API functions for setting up resources
Resource Observation | API functions for interfacing with the observe handling (RFC7641)
Resources (Internal) | Structures, Enums and Functions that are not exposed to applications
Sessions | API functions for CoAP Sessions
Sessions (Internal) | CoAP Session Structures, Enums and Functions that are not exposed to applications
String handling support | API functions for handling strings and binary data
TCP Support (Internal) | CoAP TCP 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