As of 2012 all VDGS ( Stand Entry Guidance) have to be available to both flight crew.
As a comparison at EGLL all the market leaders were installed as a trial on the old stand 215 with a view of supplying Terminal 5. Safedock came out best even though we had Papa, Mirror, Stop Arrows and FMT APIS already at Heathrow. Seimens etc did not seem to be reliable and were fairly new so there was some sceptism whether they could deliver.
As a result of ICAO that takes papa, mirror and stop arrows out of the equation. Even though a good old mirror or a bit of paint were damm cheap. A BAA requirement normally !!!!!
Safedock is more user friendly from a set up point of view and a investigation point of view as you can interrogate the system straight away without having do download log files onto a laptop as with APIS.
APIS have actually bought out a APIS ++ Add on which bolts on the side of an APIS so it gives the First Officer a facility to a park. This is being used as a trial on stand 334.
Regarding entering a stand without any SEG or marshaller this is an absolute NO NO. Any aircraft should wait in the taxiway. an illuminated SEG means the stand is clear. If you were to enter a stand and hit something ( which has happened) I think you will be looking for a new job.
If you do enter and stop half way down yes the SEG may not illuminate. It varies from stand to stand.
