as one of those responsible for championing utilities without going into a lot of detail as to why or how they are so useful there is method behind this "madness": promoting independant research - it's much more productive in the long term than spoonfeeding...
as for a definition of "utility" - simple modules that provide simple functions for multiplying, merging, derivation and modifying electical signals - not envelope/function generators or LFOs - they're modulation sources nor effects modules (sound modifiers) - envelope followers are utilities, waveshapers are imo borderline between being sound modifiers and utilities - I'd also class interfaces of any kind as utilities
in the formula in my signature lots of modules are in multiple categories - Maths is both a modulation source and an utility depending on how it's used/patched... personally I'd err towards modulation source - as that's how I generally use it - or don't really fit - sequencers are a good example... but might fit in as modulation sources or sound sources, depending on how fast they are clocked
I tend to categorise by primary intent
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities