Empirical vs. Defined & Prescriptive Process
In general, agile methods promote empirical rather than defined processes, a categorization used by industrial process experts [OR94]. A defined process (also known as a prescriptive process) has many predefined and ordered activities to be followed during development. Defined processes are suitable for predictable manufacturing domains. Empirical processes are used for high-change and unstable domains; rather than many sequenced activities, they are based on frequent measurement and dynamic response to variable events. This approach is reflected in Agile Principles 12 and 13.
For example, Scrum is silent on the activities of an iteration, other than the daily Scrum meeting as the measurement and response mechanism. The UP, on the other hand, strikes a middle way; it lists common activities (e.g., Write Release Notes), but the team is welcome to ignore or do them in any order.
Agile methodologists understand that the degree of "method weight" and predefinition of ordered activities are functions of the project type. An agile method or project lies on a continuum of more or less empirical, driven by need. A medical device under FDA approval requires more formal, predefined activities.