How does Open-Seats settings work when i make a multi-player game?

When you make a game with open seats, those seats (which are basicly slots) are not asigned to any player. If you use an open game, this means that random players can join your game. If you want to create a game and none of your friends are online, then you can use the open seats to get random persons. You can also use open seats with invite only games, this is mostly done with custom scenarios, so the invited players can decided for themselves what team (or which slot in the scenario) they want to pick.

I am slightly unclear about what you are asking.

If you are asking what an "Open-Seat" is, from the wiki: "Game creators can add Open Seats to their game in the place of a player. Any game with open seats is considered an Open Game. All open games are listed on the Open Games Tab, which is located under the main Multi-Player tab."

If you are asking how to add an open seat in the place of a player, when you are under the "multi-player" tab, first click "Create Game." This will open the Create Game Wizard. You will be directed to the "Select Players" section of the wizard. To add an open seat, click on the "Open-Seat" tab and add as many as you want. If you are not a high enough level, however, the amount of open-seats you can add may be diminished.

Finally, if you are asking about how to define open-seat prerequisites, when you are creating a game, after you are done selecting which players you want from the "Select Players" part of the wizard, you will be directed to the part of the wizard that allows you to create your game, with a default template already built in. To change the prerequisites, click on "Change Settings" then select one of the three options. Then select the "Open Seat" tab. From here, you are able to define the prerequisites of your open seats. Assuming that is all you want to change, just click "Next" then "finish" from the upper right hand corner, and you're done!

Sorry if I went into to much or not enough detail, but from your question, it is hard to tell exactly what you are looking for. Nevertheless, I hope this large block of text was able to help you!