I signed a contract with a roofer. They have started the job Today but I can see in all the plywood sheets the following stamp: "DEGRADE ALL OTHER MARKS VOID". I think that means they are using second or even third quality material, am I right? Should I immediately ask this company to change these plywood pieces for first quality ones?
I've never seen that label but I would assume they are seconds. (called blows here).
Should they be using it? Depends on what they are doing? Are they re-sheeting over existing skip sheeting? If so I don't see an issue with it. The skip sheeting still remains your main decking. The plywood just gives a solid surface to nail an asphalt roof to. Standard here is 7/16" osb and I would consider even 1/2" plywood seconds to be superior to osb.
Now if it is just going over trusses and is your primary roofdeck then, yes, I would have some concerns about it.