system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
Apical collar signifies:
assistant
The correct answer is  ΑPICΑL COLLAR = Retentive form (usually nearly 2 mm of parallel walls at apical region) 
Keyword is TUG BACK.