ARB means that is not a core feature.
every party which provides an OpenGL implementation(graphic card driver) can extend OpenGL with own extensions. You have probalby already seen some, they start with something like GL_[ATI/NV/APPLE...]_XXX or GL_EXT_XX
when other vendors decide that a extension is quite useful they make an official OpenGL extension out of it. ARB means Architecture Review Board and is a board out of a number of companies which decide how OpenGL should evolve.
After a while a ARB extension sometimes evolve into a new core feature of OpenGL.
Between these stages the API and usage of a extension can change a bit, so the first iteration of a Extension might only be usable on a Nvidia card but after the promotion to an offical extension it can be used on a lot of other graphic cards.
In your case GL_ARB_depth_clamp was based on NV_depth_clamp(written against OpenGL 1.2) and is written against OpenGL 3.1, but was included as a core feature in OpenGL 3.2.
So it might be possible that this extension is available to very old Nvidia cards with newer graphic drivers.
btw you can get all the info from the opengl registry which has an entry for every opengl extension: your example depth_clamp