Briefly, as I experience it, FEED is a part of the FEL process, typically taking place at FEL-3. FEL is more about the overall business decision making process that goes into a project, from concept, to feasibility, to execution to commissioning, including the necessary decisions to stop along the way and back up or abandon it. FEED is more about the nuts and bolts of seeing if it will work as conceived once someone HAS decided to spend money on it after FEL-1 and FEL-2. As Engineers, we typically don’t get involved until it gets at least to FEED, because we want to get paid for our work, so someone has to have already released those funds (hopefully) after the FEL-1&2 processes.

As a cautionary note; if someone wants to drag you in at the FEL-1 or -2 stage, be prepared for not getting paid for your time. Some people will do this to equipment vendors as a way to help them make their earlier decisions. As the Calamari commander in Star Wars said, “It’s a trap!”

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

FEED would be conceptual design. detailed design comes next

I have been involved in several FEL processes for a variety of projects and clients. Unfortunately, they end up going the same as projects without a FEL process. There is supposed to be a lot of upfront planning to anticipate issues and make the project run smoother. But really, it is usually just another road block to performing the real work on a project. There are usually meetings, planning sessions, pull planning, and a lot of other fancy buzz words people like to use. It seems like clients implement FEL to convince themselves they being proactive.