Current dialplan flags used by app_conference
Please note that app_conference is still work in progress, so this document might be outdated. As always, the source code is the definitive reference (member.c in create_member())

Mute/no receive options:
'C' : member starts with video muted
'c' : member starts unable to receive video
'L' : member starts with audio muted
'l' : member starts unable to receive audio

Speex preprocessing options (right now app_conference does preprocessing only for Zaptel members):
'V' : enable speex preprocessing Voice Activity Detection 
'D' : enable speex preprocessing De-noise
'A' : enable speex preprocessing Automatic Gain Control
'T' : member connects through Zaptel, so speex preprocessing should be enabled

DTMF options:
'X' : enable DTMF switch: video can be switched by users using DTMF. Do not use with 'S'.
'R' : enable DTMF relay: DTMF tones generate a manager event
If neither 'X' nor 'R' are present, DTMF tones will be forwarded to all members in the conference

Moderator/video switch options:
'M' : member is a "moderator". When a moderator quits, all members are kicked and the conference is disabled.
'S' : member accepts VAD controlled video switching.  Do not use with 'X'.

Miscellaneous:
't' : member accepts text based control messages.  The messages are described in a separate document
'N' : Assume that the member starts off with camera disabled.

Future development (these are not implemented yet):
'x' : marked member.  We plan to change the behavior so that when ALL moderators quit, all members that are marked will get kicked. Other members in the conference are not affected.

