Claims are very complex. What is it exactly you are after? Calculation of claims, legalities of claims? A general overview is that the claim is to recover costs that you have incurred that were not in the original contracted scope of work, for instance a common claim is a Delay Claim for construction delays outside of your control. If your contract was based on 6 months and you are into a project 12 months, you have overhead to support the project among other costs, you would file a claim to the GC or against your sub to recover.