User needs to display custom record type on the Job/Project record's sub-tab. Solution is linking custom field type of List/Record sourced from list Customer.
To create a child custom record type:
1. Navigate to Customization > Lists, Records, & Fields > Record Types > New.
2. In the Name field, enter a name for the record type.
3. Click Save.
4. Click New Field.
5. Enter a name of the Label field.
6. Set Type to List/Record.
7. In the List/Record field, select Customer.
8. Check Store Value field.
10. Check Record Is Parent field.
11. Click Save.
Note: There is a side-effect of this solution, which is that user sees the sub-list of custom record also on Custom sub-tab of Customer record.