This is a PRO feature (not available in the free version).

In the dialog "Custom fields" you can define your own custom fields for various modules like Inventory (articles, services and fixed assets), Orders and People.

Add / Edit / Copy [more]
Hereby you can define new custom fields resp. edit or copy existing ones.

Delete
Hereby you can delete one or multiple custom fields. Please note that if you delete a custom field, all of its stored values are gone and cannot be restored. Consider setting a custom field inactive instead of deleting it, if you don't want to lose the values.

Reorder
You can change the order of the custom fields in this dialog via drag & drop.

Variable
The value of a custom field can be inserted automatically in various places by using the variable (e.g. $customField7). For example here:

Text above / below in document (orders)
Name / description of an item (orders)
HTML of a document template (orders)

More tab

Here you can see some custom fields defined for Inventory articles as an example.

They will be available in the Add article dialog in the "More" tab by default, or in custom tabs:

Table columns

These fields will also be displayed in the table once you enable the corresponding columns in the column menu:

They are then displayed in the table as well as in the exported PDF / Excel / CSV file. They are searchable, filterable and sortable.