How to keep/save bill type as Invoice and qoute seperately

It works if your set up is correct. You need to have an Inv Prefs record set up and enabled with a description set to the invoice type (aka Quote) that you want to set up. I’ve attached images to show set up and help info.

The inv_number_starting_point shows the setup instructions in the How To… section on the left.
The inv_pregs (should be inv_prefs) the invoice preference set up I have on my test system.
The inv_pref_quote_edit shows the edit screen for the Quote type preference I have set up on my test system.
The inv_pref_numbering_group shows the help information on the Invoice numbering group field of the Inv Prefs record.
The inv_edit shows the edit screen for an invoice of type Quote on my test system. Note the drop down and how it shows the enabled Inv Prefs to select from.

By far the most complex part of the set up is if you want quotes (or other invoice types) numbered in a different sequence than the standard Invoice types. The set up for this is manual and uses the si_index table in the database.

See instructions in image file.

Are you running master_2020 version? If so, my last update was to add code to set up the index file and numbering from the INV PREF maintenance screens. I will be updating the documentation in the knowledge base to reflect these changes soon.

If you are on master_2020, download the lasted version and see if it helps.