The vtiger UI on Settings > Module Management > Module Layouts & Fields is sadly not very helpful. Beside of showing some basic details like mandatory or display order, the real custom field name is sadly hidden.
Identify the custom field name of Quotes in Module Layouts & Fields UI
If you do not know your custom field name, we have to identify it. The fastest way is to go to Settings > Module Management > Module Layouts & Fields. Now select your module at top select box, e.g. your module Quotes. Now navigate to your field and click F12 and select inspect or in Firefox or Chrome press Ctrl-Shift-C and search for the html attribute data-field-name.
Usually the custom fields are named with prefix cf_ and a number like:
cf_1234
Identify the custom field name via database
You can use the database too, if you are master of SQL.
To identify where vtiger is storing the data of the custom fields, read How to find standard & custom field locations in database?
Related articles
To change the length of this custom field, read How to change field length of custom fields?