They are either illustrations of OBJECTS, or what things ARE (nouns)…
or illustrations of PROCESS, or what things DO (verbs)…
or illustrations of CONCEPTS, or what things might STAND FOR (ideas)…
or they can be combinations, such as OBJECT / PROCESS…
or OBJECT / CONCEPT…
or OBJECT / PROCESS / CONCEPT…
Style comes into play where it supports the OBJECT / PROCESS or CONCEPT mode of message communication.