This is an interesting question. We can only surmise that must be some type of local vernacular or lexicon.  Because, as you stated, the term is not defined anywhere in the FAR, DFARS or any other acquisition of procurement regulation or guidebook. When the FAR or DFARS or other statutory or regulatory documents do not define a word or term we resort to the common dictionary definition. In this case I'll use Merriam-Webster, "planned" means: to have in mind, intend, to have a specified intention, to devise or project the realization or achievement of.

It's likely the term is used for situations when it is known that a modification will be required. Examples of such a scenario could include: exercising an option (again, as you stated), modifying a contract to add an award fee earned by the contractor, modifying a contract to incorporate an updated wage determination within the required time frame, etc., etc.