From Fedora Project Wiki

Building NSS is not accurate. It should be called Updating NSS. The section is so long that it merits a separate page and we can link to it from here. The new page should lnk back to this so readers can have some context regarding the split if they need it. Explain that this refers to major updates, that is, rebases sometimes triggered by security updates. Most updates occur at the higher levels of nss, they are new releals with downstream patches. These are simpler and have not caused breakage.