Dimension 9100 question

Related to my PCTV question, but more general:

Ever since I've had this machine (probably 3 years) it's been stowed on the
floor next to the desk so I've never really looked at the front panel when
it's running. But whilst investigating the PCTV card problem I noticed that
on a black bit of the front panel there are some LED "status" symbols that
illuminate at various times. These incluse a set of numbers (I think 1-4)
which light up green briefly and then go dark, which is presumably why I've
never noticed them before. Does anyone know what they are and/or what they
mean? I ask because when I install the PCTV card the numbers 1 and 4 remain
on permanently.

TIA,

PDR