When reading the source code there are some useful basics that are good to know and keep in mind:
'data' is the variable name we use all over to refer to the easy handle (struct Curl_easy) for the transfer being worked on. No other name should be used for this and nothing else should use this name. The easy handle is the main object identifying a transfer. A transfer typically uses a connection at some point and typically only one at a time. There is a data->conn pointer that identifies the connection that is currently used by this transfer. A single connection can be used over time and even concurrently by several transfers (and thus easy handles) when multiplexed connections are used.
conn is the variable name we use all over the internals to refer to the current connection the code works on (struct connectdata).
result is the usual name we use for a CURLcode variable to hold the return values from functions and if that return value is different than zero, it is an error and the function should clean up and return (usually passing on the same error code to its parent function).