This article describes the naming conventions used by the key variable in a key-value pair.
Naming Requirements for Keys
In Expression Builder, the name of a key variable in a key-value pair can consist of any number of digits followed by 1 (or more) letters, a dash, an underscore, and additional digits.
Prefixing Key Variables with c_
The c_
prefix is always required if the parameters that send in data on an event call URL use that syntax.