I am new to DTM and adobe analytics. I took some trainings about DTM and Adobe analytics. I am still feel pretty confuse about "could I re-use the variables". For example if I decided to assign eVar1 = productID in a property, could I use eVar1 for something else in other property?
Is eVar/ prop unique in a property or report suite? If so, what's the best way to track which eVar/prop have been used for what?
Also for all the eVars or Props, is there guidance which variables should use for which kind of event/ values?
You can reuse variables but not in the way you are thinking. Setup Data Elements within DTM to do this.
You can have a Data Element named "ProductID". Then within analytics you set eVar1 (or whatever variable) to %ProductID% and then you can use that Data Element for any other var or tool or tag that you are using DTM to deploy.
All props and eVars are unique per report suite. DTM should be setup to match the setup in the Analytics account that the property is reporting into. If you've got one site reporting into multiple report suites, you would want to coordinate variable usage.
As for your second question, props are instance variables and eVars have the ability to have persistence. With that said, I'd suggest asking the second question in the Analytics community where you'll have an Analytics focused audience viewing and responding to your question. They can likely give you a more detailed answer on the differences and possibly give you suggestions/examples.