Classifying visual aids by function: Visual aids used in teaching/extension work are broadly grouped as display aids (models, charts, specimens shown to illustrate structure), presentation aids (projected or narrated material), and design aids (sketches used to plan a message).
A model is a solid, three-dimensional replica meant to be viewed by learners to understand shape, structure or working of a real object; it is not projected or read out, so it does not fit the presentation or design categories.
By this classification, a model falls squarely under the display type of visual aid.