It basically is the command that provides the program an account it can use to access to remote database e.g sharepoint farm with a frontends/app servers and separate backend database.
The fact you are mentioning this means your are considering deploying this from ePO - therefore the instal will be handled by the ePOagent which you may have already on the sharepoint server(s).
If you install locally then an acount that has full local administrative rights on the machine is recommended to run the installer - the instaler then will provide a dialog box to provide the same information as per the RemoteSQLUser Command to provide same account. (See Page 15 - Standard Installation)
Post install if you have not specified these details or you want to change this user at any time using SETSQLACT.exe (Page 15 of Product Guide).
I would suggest if you are testing to use a farm admin account to verify all working - then the gude has an Appendix section A - where you can follow through to possibly set up an account with least required permissions (Page 97) - then once done you again can use setqlact.exe to change the account form the farm admin to new account..
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