Looking at the rest of the groups chosen illustrations and finding categories they fall into

Concept illustrations

Character 

Reference 

Specific audience 

Tone 

Realism 

Mixed media 

Colour

Dynamics 

Contrast 

Narrative 

Digital colour 

Context 

Narrative 

Detail

Text 

Deapth

Different medias

Audience specific 

Humour 

Tone of voice 

Pop culture reference 

Detail

Colour 

Choice of media 

Composition 

Technical skill

Photo realism 

Focal point 

Negative space 

Sub categories thought up after class discussion and feedback 

Content-messages 

Political 

Soterical 

Humour 

Emotive 

Process/media 

Collage 

Printing 

Digital 

3d

Mix media 

Brush work/pen work etc 

Aesthetics

Colour 

Pallet 

Texture 

Composition 

Technical

Line 

Analogue or detail 

Shape 

How it's applied 

Context 

Large scale/small scale 

Gallery 

Sketchbook

Conceptual development 

Practitioner 

Illustrator 

Agents 

Audience 

Children 

Adult 

Etc