D-Plus, what is that? What does it do?

First off, it is a 3rd party gateway addon daemon that provides:
some rather nice add on features to a D-Star Gateway system.

ECHOTESTING:
 echotest:   use "XXNYYY E" where XXNYYY is repeater call
                        URCALL: <rptr call> space  E
                       R1 or RPT1: <the <rptr> <port> your on now>
                       R2 or RPT2: <the gateway call sign and G>
* NOTE: some radio's enable you to press a key to insert "G" automatically.
Key your mic and identify or sing "Mary Had A Little Lamb"
Give the gateway a sec and you will hear the repeater key up and play back
what you have just recorded, or ECHO it back to you.

Simulcast: This is also know as an ALL CALL, Please, with due respect to all
members on the system, this is an EMERGENCY FUNCTION. IT IS NOT TO BE
TOYED WITH AT ANY TIME. You can loose privlagies if you abuse the use of it.

 simulcast:  use "XXNYYY  " to simulcast to all modules
URCALL: <RPTR CALL SIGN ONLY>

Identifacation: New to an area, do you know if they are running DPLUS?
Set the URCALL field to the repeater callsign and the last character to I.
Key your mic for TWO SECONDS, listen to the automated announcement
about the system.
   ID:         use "XXNYYY I" to recall repeater ID
*IF the system is linked to another gateway or reflector, it will also
announce that back as well.

LINKING to a REFLECTOR OR GATEWAY <HARD LINK as we call it>
This function is GATEWAY OWNER CONTROLLED. A system MAY or MAY NOT
allow roaming users to link or unlink their gateway. Check with the ID
annoucement for any details on this.
Registered users would be able to do the following:
             
   link/unlink: use "XXNYYYML" to link current module
                                    to gateway XXNYYY module M.  Use "REFNNNML"
                                    to link reflector NNN module M.
                                    Use "       U" to unlink.
                                    You can unlink one module using another
                                    local module using the repeater callsign
                                    in urcall characters 1-6 followed by
                                    the module to unlink in character 7 and
                                    'U' in character 8, e.g. "W4DOC CU".
                                    There are currently eleven reflectors,
                                    001-011.

*** Note that you should not link more than  one local module to
the same reflector/ module. This is to prevent any kind of LOOPS or
looping.

** Note that all of the callsigns above are in URCALL and
    rpt1 is local module and rpt2 is gateway.

Know Reflectors online at this time:
    REF001: US / Chicago / high bandwidth
    REF002: US / Nebraska / high bandwidth
    REF003: AU / limited bandwidth to US
    REF004: US / TEXAS / high bandwidth
    REF005: UK / London / high bandwidth
    REF006: UK / used for DE / high bandwidth
    REF007: IT /
    REF008: IT /
    REF009: US / Arizona
    REF010: US / New England
    REF011: IT /
    REF012: US / San Diego / high bandwidth
    REF013: UK / regional nets / high bandwidth

There are other ADMIN options to Dplus that are not posted here, since this
is for an informational view of the functions USERS would generally use
for normal day to day communications.