The X29 profile is just a generic profile
that can be applied in different ways for
X25 PAD connections.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fwan_r/x25cmds/wrfx253.htm#xtocid25
The username command is another generic global
command which can be used to authenticate users
connecting to the box; the associated autocommmand
can control what that user does, once connected (in
your case the user will automatically begin an X28
session).
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/ftersv_c/dcdpad.htm
The translate command will cause incoming TCP
packets, destined to 10.10.50.87 port 30000,
to be directed to an X28 session with
a maximum use of 1.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/ftersv_c/dcdpt.htm