What do the "N" and "T" labels mean on junction box cable clamps?
From the UL White Book

Guide Information for Electrical Equipment The White Book 2012
Outlet Boxes and Fittings (QBPZ)
Metallic Outlet Boxes (QCIT)
CLAMPS
Boxes may or may not be provided with clamps. When clamps are provided, the carton is marked to indicate the type of wiring system or combination of systems for which they have been tested. The clamps are marked with the following letters or combinations thereof to indicate that they are suitable for use with armored cable (&quot;A&quot;): flexible metal conduit – &quot;F,&quot; nonmetallic-sheathed cable – &quot;N,&quot; or flexible tubing (loom) – &quot;T.&quot; Clamps suitable for Type MC metal-clad cable are marked &quot;MCI&quot; for metal-clad interlocking armored cable, &quot;MCI-A&quot; for metal-clad interlocking armor ground cable, &quot;MCS&quot; for metal-clad continuous smooth-sheath cable, and &quot;MCC&quot; for metal-clad continuous corrugated-sheath cable. If suitable for all seven types, the clamp is marked &quot;ALL.&quot; Clamps suitable for nonmetallic-sheathed cable are also suitable for multiconductor underground feeder and branch circuit cable where used in dry locations.

tl;dr
The letters indicate what type of wiring system the clamps should be used with.

A Armored cable
F Flexible metal conduit
N Nonmetallic-sheathed cable
T Flexible tubing
MCI Metal-clad interlocking armored cable
MCS Metal-clad continuous smooth-sheath cable
MCC Metal-clad continuous corrugated-sheath cable
ALL All