This cap cover that little top area of the Z carriage on the A1 Mini. Helps keep dust from entering inside the lubrication hole.

Henlor's original model called for double sided tape to help secure it better, however I found adding a small tab to slide in to the hole a hair secures it enough without needing to utilize tape. I suggest removing it from the print bed once everything cools down to avoid any slight warping since the model is very thin.