Setting a variable based on another form's field that was chosen previously in the current form

Setting a variable based on another form's field that was chosen previously in the current form

I'm creating an application form and I have a lookup field that accesses another form that holds the positions and whether or not a Drivers License is requiered. I later want to restrict the field that allows you to input an id type to only drivers license if the previous lookup field chose a job that required a license. Otherwise it would allow you to select Other types of ID. Once I am able to do this I will then be able to use this technique in other areas of my main form to get specific types of information based on specific jobs. I'm a noob so please be patient with me. Thank you in advance.