It stands for Authentic Use, Restricted Use and Clarification and Focus.
Example
ARC could be used to describe the staging of a grammar presentation lesson which starts with an explanation of rules. Here it would have a different order: CRA.
In the classroom
Authentic use might be a conversation stage that is designed to include the language that learners should practise; restricted use could be a gap-fill exercise on the language, and clarification and focus an explanation of rules on the board.
Further links:
https://www.teachingenglish.org.uk/article/speaking-elementary-learners