BTE events are Business Transaction Events. These events can be used to put custom specific logic in ABAP code interface when this event occurs.
It is used by standard SAP, partner software, and you can create your own interfaces.
Pros and cons
The BTE can solve business specific issues in a graceful way with custom code. This is a large pro.
The biggest con is that the technique is hardly known. When there are issues inside the Z coding for BTE, they are tough to spot and to detect. Good documentation is required of the event usage (both on paper specification and inside the Z code).
The main transaction for BTE events is FIBF:
The screen is indeed empty.
The main parts are below Settings and Environment.
To list all the events, choose Environment, Info System (P/S):
And the result screen:
If you select a line and press the Display Act Comp button, you can see the details, including the existence of a customer implementation:
Background and references
Many good blogs already exist. So no need to repeat here.
List of blogs and sites: