marc documentation?

From: Jonathan Rochkind <rochkind_at_nyob> Date: Wed, 27 Jan 2010 14:59:04 -0500 To: CODE4LIB_at_LISTSERV.ND.EDU

I know I've seen documetnation on the LC site before for since-abandoned 
marc bib tags, like 400. But I can't for the life of me find it now 
navigating around the website or googling. does anyone know where this is?

Jonathan