The checkout/reset concept is severely and confusingly overloaded/overlapping. Addition is confusing as well, although not to that extent.
Changes can be in many states, and many operations can be applied (all the permutations of: in/off index, deleted, changed, added), but the UX has never been (re-)designed for usability, so that it's extremely confusing to remember what to use.
As I said, I have a totally different experience. I never had a day where it 'clicked' or anything, I just feel I understood the model and now it makes sense.
