There are 3 main BSA Accounts
Membership/AKELA - This saves your role in Scouting (Parent, Scout, Leader) and your Unit (Pack 23, Ship 7609). This assigns a BSA # or MID (Member Identification Number), this is a unique number that is council specific. Councils can help you change memberships in their own council.
my.scouting.org (MYST) Username - This is the user name created to log into my.scouting.org or other BSA systems (Scoutbook, Events, Training, many more.) This is tied to your MID if it matches you correctly. If no match is found in the system it will generate a new MID for you (rather common.) If you have multiple MIDs you can log into my.scouting.org > click Menu top left > Click Manage Member ID > one of the BSA # will be there marked PRIMARY > then add the other MID (BSA #) - you want to make the one that is your unit registration PRIMARY > whatever MID is primary that is the identity you will have logging into any BSA system. So, if you have a parent MID, and a Cubmaster MID - if Parent is set primary and you log into Scoutbook, you will have no leader abilities in Scoutbook. It also means most of this you can control, if there are name variations, that is often an issue but Council or SUAC can help with those. The system does not like emails for these, and having more than one with the same MID can be troublesome.
Scoutbook User - this is your Scoutbook account - all accounts have a UserID # - this is a unique number that holds the history of the account. Users are never deleted, rather they are hidden from you if you are not connected. Councils can merge users within their own council, or National or SUAC can merge all users across councils.