Slight clarification on ZZTong's comment about the parts. The same name you use is for the POIMarkerGroup (not to be confused with the part name). Any parts in the same group will spawn only one of them based on probability that you set for each.
So if you have 4 markers... the first 2 are group1 and the second 2 are group2, all are set to PartSpawn for type, then you have part1, part2, part3, part4 as the parts to spawn, and probability of 0.2, 0.3, 0.4, 0.5 for probabilities of those...
From group1, you will spawn either part1 or part2 or no part, and from group2 you will spawn part3 or part4 or no part. The probabilities will determine the chance of which spawns. It used to be that if you had 0.2 and 0.3 as the only probabilities in the group, you'd have 20% chance of the first part and 30% chance of the second, leaving 50% chance of no spawn. I heard that this may have changed and now the probabilities are added together and then use that value when determining the chance to spawn, but I don't know the specifics of that. I think ZZTong was the one who mentioned that, so he probably can explain it better.
That's a good point; I was speaking more casually when the details do matter. So here it is in detail:
This is showing three billboard Parts. They're all placed in the same location so the Prefab Editor is showing them sort of all together. When you're looking at those boundng boxes, note that each billboard in this example has the same footprint but they are different heights. (They are shown next to a POI Marker, which is that partial area in the lower right corner.)
I selected one of those billboards and brought up the POI Marker Editor (dialog box) which knows how to do POI Markers, Parts, and Road Exits. One of the billboards was selected, so the details describe it. I'll describe it next.
The Type is Part. The list shows all Parts right now because I use a lot of Parts, but if you were to scroll through the list you would also find POI Markers. Other things on that list include cobblestone pallets, a wall, two highway transitions, a horse trailer, a fallen log, and a portapotty. (This Tile involves highway construction.) So not shown is a section of road that can either be complete or under repair.
Each Part is made like a POI, tagged as "part", has a filename that starts with "part_", and appears in the Prefabs/Parts folder of the game or a modlet.
Once included on a Tile, you give that placement a "Group Name". The way it behaves depends on if the Part's Group Name is unique or not.
Unique: If you have one part with a unique Group Name then the "Part Spawn Chance" is the probability that the Part will appear on the Tile. So if this were the only Part with a Group Name of "billboard" then it would have a 0.4 or 40% chance of appearing.
Not-Unique "Current Way": These days, it appears if there is more than one Part with the same Group Name that RWG will pick one. In this case, there are three Parts with a Group Name of "billboard" and I happen to know each has their Part Spawn Chance set to 0.4. The way I believe it works then is the probabilities are totaled and then a random selection is made. So 0.4 + 0.4 + 0.4 = 1.2, so a random number between 0.0 and 1.1999 is determined and if the random number is 0.0 to 0.3999 the result then the first billboard is used. If the random number is 0.4 to 0.7999 then the second billboard is used. If the random number were 0.8 to 1.1999 then the third billboard would have been used.
If you wanted to favor one billboard, give it a higher Part Spawn Chance. So, imagine chances of 1.0 + 0.5 + 0.5 ... Then it's a random number between 0.0 and 1.9999 and the first POI has twice the chance as the other two.
Not-Unique "The Old Way": The way I believe the Not Unique approach worked before 1.2 or 1.1 was that RWG would loop through each Part with the same Group Name and roll the dice. So with the three billboards...
Use the first billboard? 0.4 is a 40% chance, so roll the dice. If 0.0 to 0.3999 then use it and exit the loop, otherwise, try the next one.
Use the second billboard? 0.4 is a 40% chance, so roll the dice. If 0.0 to 0.3999 then use it and exit the loop, otherwise, try the next one.
Use the third billboard? 0.4 is a 40% chance, so roll the dice. If 0.0 to 0.3999 then use it and exit the loop, otherwise, try the next one.
Oh, there isn't a next one, so don't use any of them.
That approach made it possible to have a group and still have empty space. That doesn't seem to be possible now, unless you specifically make it possible. That is, with the current approach, you might make a Part that contains noting but a piece of deco trash. Then put that Part into the same Group and give it a chance to appear.
