Here's what I have found:
There is a place where you can enter shortcuts for text to auto expand & it can function as a custom dictionary as far as spell check goes. In the beta Apple has "omw" as a shortcut for "oh my way", so when you type "omw" it auto expands it to "oh my way". You can put a word in without a shortcut and spell check ignores that word. One of my gripes of iOS is it will auto correct "hell" to "he'll" so I entered "hell" as a word without a shortcut and now iOS doesn't auto correct it or show it as misspelled.
Hope that is what you are talking about.
EDIT: Oh well, beaten to the punch.