it says to name each essay with the name of the school that you are going to send it to. if you have a generic essay, then you do not need to name it specific to each school.
Here's what OptomCAS says. It really helps to just read the site
"After you save your essay(s), you may upload them to be sent to the appropriate designated schools in the "School Designations" section found at the end of the application. We encourage you to name each essay to correspond with the name of each school. "