Can you use PFI with employee/manager self serve?

Depending on the self-service task, there is some approval/workflow. For example, a manager can initiate an action, like a promotion, from MSS, and it will go to the approver's inbasket. However, for most tasks, e.g. employee address changes, it is "live" entry, and doesn't have an approval process.

How do you know which fields have an approval? Is there a document or table that lists them somewhere?

There is a document available on Lawson's support site under the ProcessFlow documentation section, called "ProcessFlow Reference Guide" which details each delivered service for the applications release you are running.

A ProcessFlow service is triggered by the backend application form.

For example, when a Personnel Action is entered in Manager Self Service, it calls the backend application form PA52, which then triggers a service called "Personnel Action Approval", and passes various information. In ProcessFlow designer you would create a flow to react to that trigger and perform whatever logic you want to happen. In ProcessFlow administration you would attach that flow to the service. (I'm really trying to keep it simple!)

There are sample flows that are delivered with ProcessFlow for some of the services. In this example, there is actionapproval.xml and actionapproval2.xml.