I've recently been interested in missions that start other missions while the player is in flight, mostly for special ships.
I've been using Sxxx in the OnShipDone field for this, usually set to Observe something that entered the system after the mission began. All mission ships were set to appear in the initial system.
I had the idea in my mind to see if 'Child Missions' inherited certain data from the 'Parent Missions', such as the Return Stellar (I'll use RST for short here).
I can't remember seeing anything about this subject on the forums before, nor did I find anything related using the search, so I decided to experiment with it and report.
When 'Child Missions' used -4 ("Where the mission began") as the RST, they used whatever planet the 'Parent Mission' began on. It didn't matter if there were multiple planets in the system, or if the planet was the first nav-point in that system or not. The 'Child Missions', as long as they had "Where the mission began" as their RST, inherited the correct planet.
For a follow up, I looked at a successive chain of 'Child Missions', one of which used "Random Inhabited Planet" as the RST to see if the following missions inherited that.
What happened instead was that missions following that break in the chain selected the first planet in the current system as the RST, whether or not that had been the planet the first 'Parent Mission' had been accepted on.
When I looked at the case where the first 'Parent Mission' had a different RST than where it had begun at, the 'Child Missions' still pointed to the planet where the 'Parent Mission' had been accepted as their RST.
When I looked at a successive chain of missions using "Random Inhabited Planet", the result was a series of random planets for each mission.
When I moved the 'Parent Mission' ship to a different system from the first, but had it immediately Sxxx another mission whose mission ships appeared in the initial system, again those 'Child Missions' used the first planet of the starting system as the RST.
When I looked at when the 'Parent Mission' had the mission ship in an adjacent system, and had to be Observed before starting the first 'Child Mission,' the Child Missions used whatever the first planet in the adjacent system was.
When I put the first Parent Mission ship in an empty system, and had the Child Missions start from there, the Child Missions consistently pointed to the Spöb with RID 128.
In conclusion, as long as the succession of missions is entirely within the system that the first was accepted in, and if all the Child Missions all used "Where the mission began" as their RST, then they all pointed to the correct planet that the first mission began at as their RST.
If the mission succession moved outside of that system, then the Child Missions used whatever the first planet in system the Child Missions were started in as their RST's. Same as happened if one of the missions in the chain of succession pointed to some planet outside the system as the RST.
If the Child Missions started in an empty system, it pointed to whatever planet had RID of 128.
When the Child Mission is started in a system with only an uninhabited planet, or a planet that couldn't be landed on, the mission still used that as the RST. Even in cases where inhabited planets could be landed upon were in the same system, it used the uninhabited first planet in the system as the RST. Be careful.
If there was no planet listed in the first nav-slot for the system, but there were in the second or higher slots (-1 in NavDef1, but others in NavDef 2 - 16), then the next planet listed by the system was used as the RST by the Child Mission.
The order the planets are listed in by the system is important, but not position in that order. So, the RID's of the planets didn't matter, or if NavDef's 1 - 15 were empty. The first planet listed by the system, even if it was in NavDef 16, is what the Child Mission pointed to as the RST for "Where the mission Began", when it hadn't inherited a valid one from the Parent Mission.
Since I haven't seen this particular subject come up before, I thought it might be of interest to post it here.