What is the difference between Secure Subpages and Subpages with Display Conditions?
When you select Secure Subpage, View and Edit rights are generated in the system for that subpage. You then have the option to select who can/cannot access that subpage and whether or not the specific resources have View Only access or Edit access.
With Display conditions, you can define the specific conditions under which the subpage will display. There is a different kind of flexibilty with Display conditions as you can define the display conditions so the subpage only displays for resources within specific groups or based on the value in a specific field, etc...
Using Display conditions does not give you the ability to display the subpage in a view only capacity for some users while still editable for others.
For general purposes, if an attribute is only displayed on a subpage that is Secured or has defined Display Conditions, and the attribute is added to the object list view, it will only display a value if the resource has appropriate rights and/or the data meets the display conditions. This is referred to as 'Attribute Value Protection' (AVP).
For more information on AVP, reference the following article:
TEC1272884 : How to Configure AVP Settings to Improve Performance