Since Invoices plugin version 4.2.4, a new feature was added - Invoices and Expenses custom fields. Let's take a look at the following example of the new feature.
Creation
To create them, please go to administration (on top) - custom fields - new custom field. Here, we may select any type of custom field. For the particular example, let's choose the "issues":
The feature itself
The new feature itself comes here - when you choose the format of the custom field. Please check below.
We created the following custom field with format "invoice" (in a similar way is the "expense" format):
Appearance afterward
In the issue afterward, you could select existing invoices from the drop-down menu of the custom field.
In the below screenshot, you could see the applied invoices field with 3 invoices added and the expenses field with 2 expenses added.