This is used as the initial model in the stepwise search. This should be either a single formula, or a list containing components upper and lower , both formulae. See the details for how to specify the formulae and how they are used. If the scope argument is missing the default for direction is "backward". Larger values may give more information on the fitting process. Typically keep will select a subset of the components of the object and return them. The default is not to keep anything. The default is essentially as many as required. It is typically used to stop the process early.
AIC as practiced is appealing to base human instincts to rank and to be wishy washy about inferential frameworks. There is NO philosophy of science that says ranking models is important. Its barely better than useless to science.