> In the Cambridge Grammar of the English Language, the authors generally start by cataloging the forms in use, then describing how those forms are used
So for example, walk is what they call the plain form of the verb. They explain that this form is used in imperative, infinitival, and subjunctive constructions.
They would call it the imperative-infinitive-subjunctive form, but that's a bit too wordy, so they went with "plain" :-)
They don't posit three separate forms (imperative, infinitive, subjunctive) because these forms are always identical for all verbs