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Sub-type
A sub-type is a more specific value inside an annotation type. The type names what you watch for. The sub-type names which form it takes. "Gaze" is a type. "At student" and "at board" are 2 of its sub-types. You choose a sub-type when you make an annotation on a track.
Example: A coder studies where a teacher looks. She uses the type "Gaze" with the sub-types "at student", "at board" and "at screen". Later she can count each one separately.
Why it matters in GRAPE MARS
Sub-types let one type hold many specific values. You get 1 track for "Gaze" instead of 3, so the timeline stays short. Your charts can still split the results by sub-type when you want that detail.