What?
In the real world, to receive bookings globally through travel agencies, and airline needs to be present in one or more of the (few) Global Distribution Systems. The airline also needs to participate in the so-called Billing and Settlement Plans for the countries in which they want to sell.
Both participating in a GDS and a BSP incurs additional costs. GDSs require a setup fee as well as fee on every booking made through them. BSPs require a “buy-in fee” that (in AS) depends on the size of the market they cover and potentially a perpetual fee as well (subject to game design considerations).
There will be different GDSs a player can chose from, each with slight regional differences. So get perfect global coverage, an airline would have to be connected to more than one GDS.
Whether and which GDS can be connected is determined by the choice of Passenger Service Systems (PSSs) a player makes.
In the real world, the primary motivation for BSPs is to simplify accounting and billing between agents and airlines. We might model some of that was well to give the otherwise very convenient GDS distribution channel a bit of a disadvantage (like receiving payments for booked flight only once a week per BSP or something similar).
Initial buy-in when a new airline starts will likely come with a discount or a “free budget” similar to the Immediate Delivery Program for aircraft leasing.
Why?
Especially the BSPs are very interesting from a game design perspective, as they can serve as “market protection” to some extend. To get access to the full market in a particular country, an airline needs to buy into the respective BSP. This grants a small level of protection to the airlines already in that BSP (especially when it’s their home market).
We might (and likely will) deviate from reality a bit in the way we design the costs here. They must follow game design considerations. For example, large BSPs (like for the US or China) would be more expensive than those for smaller ones. Therefore, if an airline wants to start out in an attractive BSP like the US, even if it’s their home market, they need to spend a fair share of their initial capital for the buy-in. Capital an airline starting in a smaller, less attractive market could invest into planes instead.
There will also be running costs and possibly costs scaling with airline size, to prevent large airlines to easily be able to just buy into all or many BSPs without a noteworthy disadvantage.
When?
Requires a fleshed out Distribution System and would ideally build on top of Passenger Service Systems (PSSs).