The "Custom fields" menu allows you to add a field in the creation of a piece of equipment. This field can be mandatory or not, and you can include any information that may not be present in the equipment's creation page.
It is not currently possible to delete a custom field once it has been created.
However, it is possible to choose to make it invisible, so that it won't be present in the equipment's datasheet.
To do so, simply click on the "Custom Fields" button, from the left side's menu, and then click on the eye symbol next to the field you no longer wish to use.