According to some forum posts the sliders shared there have a certain difficulty level. For example "my custom hof sliders"...yet they are custom.
My guess is that there is only "custom" but without a level set...the values themselves create the challenge. In my opinion there is only rookie, pro, allstar, superstar, hall of fame and custom but not custom settings for all default difficulty settings.
When you download custom sliders from 2kshare the level is not listed...so I really guess it does not matter if you switch to hall of fame or pro before editing sliders. No matter what you have first, it always becomes the same custom slot. For example, starting on pro, then pushing everything to 80 should result in the same slider as starting on hall of fame, then pushing everything to 80...
Now I want to know if I am right or wrong. Anybody knows?