Custom fields, which are associated with products, can be used on sales and purchase transaction forms.  

There are no limits to the number of custom fields you can have.  You can not currently search or filter by the custom field.

To set up custom fields, go to Settings under the Setup Wizard (gear icon on main menu).

Under the General tab, enter that name you want for the custom field, then click on the plus sign.

You can also edit an existing custom field label, by typing in a new name, or deleting the field.