Two basic types of release plans that are widely used are scope-boxed release plans and time-boxed release plans.
“In a scope-boxed plan the work that the team will do is defined in advance, but the release date (time) is uncertain. In a time-boxed plan the time and release date is defined in advance, but the specific work that people will do is uncertain and is being elaborated with time.
“Time-boxed plans are the most preferred way of release planning. This will constrain the amount of work and people will automatically start doing the prioritization which is the fundamental concept of Agile.”