Yes that is true, some customer defaults control Session settings, which means that changing the default will effect all subsequent sessions of NX, while some are Part specific and therefore will only effect new parts created from scratch. We refer to this as the 'Scope' of the default.
Granted, there are some settings which are ostenisbly Part settings, but which there are options that will cause existing parts to be updated as well. I think you will find that many, if not most, of these defaults are in the Drafting area.
Now how do you find out which default item's Scope is a Session and which is the Part?
The easiest and quickest way to do that is to go to the Customer Default dialog and select the 'Find Default' option (the 'binoculars' icon near the upper right corner) and enter the term 'all' (without the quotes) and when the list is generated, scrowl to the far right-hand side of the listing and you will see a column titled 'Scope'. For each Default settable in NX, you find an entry indicating whether its Scope is 'Session' or 'Part'.
Now if you would like something a little more permanent to refer to, you could always export a copy of all the Customer Defaults settings to a spreadsheet, but be warned, this can take several minutes to complete. As of NX 5.0.3.2, there are a total of 9930 different Customer Default entries (and I won't even tell you how many there are in NX 6 except to say that it's a LARGER number).
John R. Baker, P.E.
Product 'Evangelist'
NX Design
Siemens PLM Software Inc.
Cypress, CA