I would like to use an attribute that is in my application (JDBC
database) to set as an initial password.

This attribute is not part of a normal modify event but I would like to
trigger this, but can I use this attribute in the operation despite the
attribute not being in the modify event.

I am trying the workout what the difference between the following are:

source attribute
operation attribute
destination attribute
any other one more applicable that I have missed (please let me know)