What does firm and not firm indicate when running a production plan in SCM

We are making a production and manufacturing plan and wanted to confirm, firstly what is the use of firm/not firm status and secondly if it has any impact on the production plan?

We had initially run a plan with half items marked as firm and other half marked as not firm. Our understanding was that firm items that have been released would remain the same when the plan is re-run with changed parameters. The re-run plan output was completely new as if a new plan was run. Is this how the system supposed to behave?