There is limited information about the real time usage of the collections in the web, but found the below 3 links which might give you some additional information to understand the role of Collections in Pega.
I have few queries related to the Collection rule - I can see the below options (i) When applied by Rule Collection (ii) When Invoked procedurally on the Declare expression rule. So what is the interlink between Collection rule and Declare Expression rule.
I had gone through the below link but I need more clarification. Please help me.