Bear in mind we are talking about correspondence chess here where reference materials are, and always have been, the norm. Top levels CC players justify engine use because, as they point out, this type of chess involves delving much deeper into a position than it is possible to do in OTB chess. As such, in their view, engines are just another tool. I am not talking about them but your average CC player.
Here’s the rule for almost all correspondence sites:
While a game is in progress you may not refer to chess engines, chess computers or be assisted by a third party. Endgame tablebases may not be consulted during play but you may reference books, databases consisting of previously played games between human players, and other pre-existing research materials.
OK, so you can’t use openings lines from a computer tournament; that part is clear. It’s the last part that’s of interest… other pre-existing research materials. Exactly what does that include? These days it is not unusual for top level players to use, and even some opening books have, engine generated lines. If I’m using Nunn’s Chess Openings, for example, and play a line that has been engine generated (of which there are a few in the book), is that legal?
Almost all commercial databases contain some games involving computer input. Thus, any database of top level correspondence chess will contain some information generated by engines. If you play one of those lines, is it legal? This is pertinent because these days some opening variations go 20-25 moves deep. What if I am following my own unpublished opening research where I have used an engine to generate lines?
Which brings up another question. On most CC sites where I play my rating is ~2300 and it won’t go any higher unless I manipulate it by playing only opponents who I’m reasonably sure I can beat. Let’s say I decide I want to go for an official CC title. Since most players rated over 2400 on all sites are, in fact, using engines, what if I play one of them? Is it right to level the playing field by switching on Fritz? If it’s OK to use an engine against other engine users, where is the cutoff point where you say you won’t use one because it’s unlikely your opponent is? I guess it all depends on what you are trying to accomplish. If you want a title, you have to be ruthless and win any way you can. If you are playing for fun then losing to a 2500 rated engine user really doesn’t matter.