For my own writing I’m defining these terms:
Device: A single set of actions with a specific outcome. A complete technique that ends with a wound as opposed to a simple description of a cut or parry. This is usually from the perspective of one fencer, with the other just there to set the stage.
Play: A deeper exploration looking at the actions and opportunities of both fencers. A play consists of several devices, often with each device defeating the one that precedes it.
Routine:A long sequence of devices strung together. It may be practiced both alone and with a partner, but either way it is from one fencer’s perspective. Assault or “assalto” is preferred by the Bolognese-Dardi tradition; however that word is also used to simply mean attack. I prefer the term “kata”, but a lot of people misunderstand what that means.
A routine, assault, or kata must be usable in a martial context, which usually means putting your opponent into a reactionary mode (German nach). There is no point in studying a routine “just to learn the guards and cuts”.