Your Custom Lists are just generic lists of Sets and/or Parts. You can put whatever you want in there and then perform various operations on them. For example:
- Create a list of Parts you want to buy
- Copy a MOC's Parts into a list and then modify it
- Import your own LEGO Digital Designer model or other file format and then Build the list to see what Parts you are missing
- Keep a Wishlist of Sets you would like, and then share it with someone likely to buy them for you :)
- Keep a list of MOCs you want to build next
Each Custom List can be made public so anyone else can view them (they won't need a login) once you share the URL.
(NOTE: in v2 these were called Private MOCs)