I am using QB online for a construction co. We are setting up all our different job locations in projects. We are entering expenses and income for for each project. My confusion is with categories and classes. I need to know if the expense I entered for example is for site work or concert or steel etc. I have about 20 set discriptors I use and those may have numerous sub sections. ie: concrete with a sub section of Rebar. 
Should I use classes or categories? When I enter an expense it is asking me for both which in my mind are the same thing. The class limit of 40 for me is a problem. Should I just turn off classes since I’m entering everything in projects and only use categories?
Thanks

Mike