xxx-add is css for playing during the css is adding onto the html (if stransition is 0 , the css will not be able to run)
xxx is the css when you give the html has "xxx" css (onOff = true). the style will keep.
xxx-remove is css for playing during the css is removing from the html (if stransition is 0 , the css will not be able to run)
As you can see, ngClass support "add" and "remove"
So if you class name is "xxxx" then your aniation class are:
xxxx-add and xxxx-remove
xxxx-add for the animation of adding your css on to html.
xxxx-remove is the animation which will be played when your css is removed from html.