The Facebook API can currently only fire standard events. However, some ad accounts have custom conversion events for which they optimise for.

For example, I am working on an ad account with 'High Quality Super Lead' that they optimise for as a conversion event. This allows them to filter standard leads, and those that are highly qualified based on their form submissions. But with the current GHL setup, we cannot send a conversion API to that specific custom conversion event, only the standard 'leads'.