System

A collection of objects that are analyzed together is College Board's own wording, from the appendix Vocabulary and Definitions of Important Ideas in AP Physics. The appendix adds the part students skip: there is no single right or wrong way to group objects, but the preferred grouping is usually the one that simplifies the analysis.

AP Physics 1 Topic 2.1 sets out the properties. System properties are determined by the interactions between the objects inside the system (EK 2.1.A.1). If those internal interactions do not matter to the behavior you are modeling, the whole system can be treated as a single object (EK 2.1.A.2). Systems may exchange energy or mass with their environment (EK 2.1.A.3), and individual objects inside a system may behave quite differently from the system as a whole (EK 2.1.A.4).

Constant and conserved are not the same word. The appendix works this out with a cereal box that tears and spills. Include every piece of cereal in the system and the amount inside stays constant. Include only what is still in the box and the amount inside decreases, yet the cereal is still conserved: it was transferred out, not destroyed. Energy behaves the same way. A falling box on its own gains kinetic energy, so its total is not constant, while the box plus Earth system keeps a constant total. Energy is conserved either way, which is EK 3.4.C.1.

So choose a system is an instruction to decide what you are willing to call external. Momentum conservation needs no net external force; mechanical energy conservation needs zero work done on the system and no nonconservative interactions inside it, a split the isolated system entry works through.