The travel benefits that employees receive vary greatly from airline to airline - I'll give you the example of the major US carrier I was once affiliated with. And since this note is intended to give you a general idea, every fine nuance and detail may not be in place

Employees received unlimited travel benefits from the first day of hire for his/herself. Also added to their "list" of eligible travellers were the spouse (if married), the parents (if alive), and children under 18. Children over 18 still living at home could remain on the list if a full-time student. My carrier allowed the spouse's parents to be named on the list if the employee's parents were no longer alive.

These people had unlimited priveleges. A service charge was paid for each trip, via payroll deduction (an example might be $20 for a coach domestic trip, $70 for a round trip in F). International trips were also available at higher service charge costs.

My carrier provided ten "buddy passes" per year. These cost $50 service charge per round trip. They could not be upgraded. If for the lower 48, the person being given the pass could travel by themselves. But for international trips, my carrier required an employee to "escort" or accompany the buddy.

The service charges were paid via payroll deduction, typically a few weeks after the date of travel.

For all travelers, seating was prioritized by the employee's year of hire.

On my carrier, no "regular" passenger willing to pay for a seat (and meeting all other requirements and deallines, such as airport cut-off times etc) was ever denied a seat over a pass rider paying passenger. Other airines differ, but on mine, space available truly meant just that.

Pass riders were always cautioned against trying to pass ride to popular destinations (Cancun during spring break is a classic example). One co-worker was once stranded in London for six days during the height of the summer vacation and ended up buying a $1000+ ticket to get home.

Some employees pay an "imputed income" tax based on a percentage of the Y fare of the trip taken (this tax also applied to the buddy's travel).