The account string is one of the basic elements of performance management and tuning in Teradata. It doesn't have anything to do with a user "account" aka logon.
The '$L' sets the Performance Group to "Low". The default performance groups are Low, Medium, High and Rush. They are used to develop a balanced performance profile as part of the TASM tool set.
These are typically set in a Profile, and users of a similar like are assigned to a profile, leaving one place to manage attributes rather than on an individual user basis.
Databases don't need for an Account String, they are a user centric construct. The reason the option is available is that the only difference between a user and a database in Teradata is a password.
If you give a user perm space, they can not only logon with the uid/pwd, but can create tables in that perm space.
Creating a database is simply the assignment of PermSpace with no password. You will need to grant privileges to any users that need access to the tables in that perm space.