title: Magento Admin formfield multiselect selected
body: <p>I´m currently developing a custom module for magento thats going to list employees.</p>
  
  <p>I have figured out almost everything. The only thing I got left is how the selected values is going to be highlighted. </p>
  
  <p>The problem I´m having is for the backend.</p>
  
  <p>I got 2 tabs per employee, one for employee data and one tab for magento categories.</p>
  
  <p>1 employee can have 1 or more categories. </p>
  
  <p>The database table that the categories are stored in are a non-eav table.</p>
  
  <p>So my question is 
  What in a multiselect determines which values are selected? As it is now, only one value is selected.</p>