To help visualize which API to use and where it is used for we have created two figures:

The role of each API

APIs and high level purpose (from left to right)

• Charge Assist API - Create App for EV charging
• Session API - retrieve updates on a charging session
• Meter Value API - get values from the Charge stations (kwh, V, Amps etc)
• Charge station notification API - get raw OCPP messages from the CS
• Location API - get information and availability from Locations
• CS management API - create and update Locations, Charge stations and EVSEs
• CDR API - Get CDRs
• Smart Charging API - Manage SC groups and charging profiles
• CRM API - Manage Drivers and Tokens

What API do I need

Another way to look at it is this decision tree.
Note: It is simplified, in many situation you may need multiple APIs

Click on image to enlarge: