How to get my need annotation type?

In article “How to Have Strainers Tag With a Bubble like Control Valves”, there is a class “STRAINER” . in the class, Add a new property and name it INSTRUMENT_BUBBLE_CLASS. Add custom attribute 'Property Custom Attributes' on this property and add the default values “INSTRUMENT_ANNOTATION”. It seems that get annotation type of INSTRUMENT_ANNOTATION.

Why do like this can get anntotation type?