Simply by the fact you created a number makes it available as a DNIS number.
For example: ext 1234 would be considered by the carrier as a DNIS of 1234.
So, in effect, a system speed dial is a DNIS as is any extension, key appearance or ARS Digits Dialed.
In some systems you have a special form to program DNIS. As a call comes in on a PRI trunk it looks up the DNIS sent in a table and routes it to the appropriate location. It's not done that way on a 3300. Any programmed number is effectively a DNIS.
Ralph