This will be domain/IP/MAC (MAC restrictions are available ONLY in ionCube Cerberus editions) or any property option where the "Field" value is "Yes". A field type set to "Yes" is one where a visitor has to enter information to create their license. For example:
For example, lets assume a product has been added and I have 2 field properties with the "Field" flag set to "Yes".
This means that a visitor is required to enter their name and product key. Lets take a look at the view the visitor has when logging in to create their license:
Note that the "i" icons are what load the help topics. So, each help topic gives a visitor some information on what to key in when creating their license.
When loading your product from the products drop down on the admin help topic page, we see the fields applicable to the visitor view.