Change extension number problem in GUI

This is my fisrt time play with Asterisk.

I am using the below version and I had found when create extension 300. It cannot create and I found it can some setting in guipreferences.conf in there it had below setting. But I don’t know what mean like ue_start, mm, vme, rge…etc. Where can I find the explanation? Any expert can help me! Many Thanks!

Asterisk Build:
Asterisk/13.1.0
Asterisk GUI-version : 2.1.0-rc1

guipreferences.conf
ue_start=6000
ue_end=6299
mm_start=6300
mm_end=6399
vme_start=7000
vme_end=7100
rge_start=6400
rge_end=6499
qe_start=6500
qe_end=6599
vmg_start=6600
vmg_end=6699
disable_extension_ranges=no
config_upgraded=yes
config_gui_version=SVN–r
default_dialplan=DLPN_DialPlan1

See viewtopic.php?f=1&t=94796 which should be the very first thread in this forum.