Sometimes, the database schema must be “manually” updated to add new primary columns to the users table, to add columns to table attributes' underlying tables, to resize existing columns or the like.
Generally it is not advisable to actually do this by hand as this can be an error-prone task that needs to be performed individually for each deployment.
Every UM plugin has its own database schema level counter that implicitly starts at level “0”. (The value of this counter is internally kept in the database and managed by the CSE class PluginSchemaMigration.