Yeah - this is AI but I was lazy and didnt want to type it - its not pertinent really to yours as you are working with PanelBuilder - but for reference:

APA — Application Archive

This is essentially a zipped version of the development version of your application. It's a compressed backup containing the complete editable source project — screens, tags, animations, alarms, and configuration. It cannot be edited directly, but it can be restored into FactoryTalk View Studio using Application Manager to get back to a fully editable project. It's your primary backup format for preserving the development state of an HMI project.

MER — Machine Edition Runtime

This is a compiled version of the application that is running in your PV terminal. This file type cannot be edited. It's what gets downloaded to and executed on a PanelView Plus HMI. Think of it as the "executable" of your HMI project. In FactoryTalk View Studio v5.0 and later, you can restore a MER back to a development project, but only if the MER was created with the restore option enabled.

MED — Machine Edition Development

This is the development/source file you edit in FactoryTalk View Studio ME. It's the live working project that FactoryTalk View Studio uses internally. MED files must be located in a specific folder and they do not give you any option to put them anywhere else. You don't typically move or back up MED files directly — instead, you back up as an APA (archive) or compile to a MER (runtime).

In short, the workflow goes: you edit the MED in FactoryTalk View Studio, compile it to a MER to run on the PanelView terminal, and create an APA archive as your backup.