Can you use a rash cream on anything or do you have to know what it is first?

Some medications/creams are meant for certain things. There are creams like Vaseline, which help cure all kinds of rashes. It would be helpful to know what rash you have, because some of those special medicated creams work really well. I had egzema and my hands looked burned (seriously). Vaseline helped me, as well as special creams by doctor gave me.