What is the role of Mail.MSMessageStore ?

To a Windows 10 Build 1909 Desktop PC where I reinstalled the WLM program , I successfully moved over my WLM data contained in "Storage Folders." This was accomplished via the Import function of WLM from the USB drive (NTFS format) where the data had been transferred before the Windows 10 Build 1909 Reinstallation had to unfortunately be performed yet again on the same machine. (Much THANKS to   ¡Firedog  and  ClitheroeKid  for their numerous previous answers which continue to guide me!!!!) QUESTION: What is the role of the data-laden Mail.MSMessageStore ? What "opens" it?  Should it be added directly to  my new WLM too? If so, how and where? Also, the WLM folder on the USB has TWO OEA Account{letters-numbers} folders ,neither of which match the single  OEA Account{letters-numbers} folder now nestled in my C Drive's WLM background folder. QUESTION:  What is the significance of the OEA account folders? Are they simply vestiges  from previous transfers?  Should they be added too? If so, how and where?  POINT OF INFORMATION: I did not move over the "WML" folder in its entirety because my plethora of subfolders were once multi-multiplied during a previous attempt.