Buddy Pass travel and Companion travel are two totally different things...
Companion travel is available to employees who do not have a spouse that they want on thier flight benefits so they can name someone else who will have unlimited stand by travel priviledges on any Delta or Delta Connection flight at the same boarding priority as the employee - Travel Companions are not eligible for ID90 travel on other airlines or codeshare flights...the Travel Companion must pay a fare, but it is less then a Buddy Pass fare...
Buddy Passes are given to employees each year to give out to family and friends - we get 8 a year - and they are only valid for 1 year from date of issue...if you extend travel past the 1yr mark the passes become invalid and refundable...Buddy Pass travel is not available on other airlines or codeshare flights, only on Delta or Delta Connection flights..so give up on going to SYD on a Delta pass...your trip to India and Frankfurt would be possible, the employee who provides you the pass can check flight loads for ya...If you are traveling on a thru flight (same flight number all the way to the destination) it is possible to be cleared all the way thru, if only cleared to the transit point you would be at the top of the list at the transit point based on your priority (S4 for buddy pass) but you still would not be above employees...

As with any standby travel, seats can be assigned 45minutes to departure time depending on the loads on the flight (I try to clear as early as possible to eliminate the bugging questions)...

Never buy a buddy pass from anyone, only agree to pay the yeild fare...if you buy a buddy pass from someone and they get caught and terminated you would be stuck without travel and would have to purchase a full fare ticket...

As for restrictions the one that most Buddy Pass riders (and some employees) have trouble with is the dress code...
http://www.deltadt.com/dlnonrev/TravelGuide.htm