Customer is a SONM user which buys and uses the computing resources of other users (Suppliers) through the system. The purpose of the Customer is to perform a specific task in an optimal way (most quickly, most cheaply, by the price / speed criterion, most reliably, or others).
Every Customer should have his own Ethereum account, that is his unique identifier.
For Customer main algorithm of using SONM is:
Full description about getting started as a Customer is here.