A variable is a key/value pair that is available to filters, rule sets and mappings and in templates at run time while processing a lead.
Every lead is born with a standard set of variables, and additional
variables are added at run-time as leads are processed by each step in
the flow. The full set of variables are stored on every event (in the
vars property) generated during lead processing.
Variables are stored on events as nested objects and are referenced at runtime using dot-notation. There are several top-level variable prefixes that logically group variables.