The way you've phrased this question is quite odd:
"on p9, you can choose p10/p11 or p10/12. Regardless which you choose, P11 or P12"
So I'm on page 9. I have options. I'm not sure if I have two (p10/p11 + p10/12), three (page 10, 11, or 12), or four (p10, p11, p10b, 12) options. I might guess two. But the next part is more confusing. I don't know if I'm going to page 11 or page 12. And what is with page 10? Is that sequential? Why is my option on p9 if I ALWAYS go to p10?
Anyway... all that aside...
You can
create a variable in your story properties, up there at the top menu, where it says "variables." Since I don't know what's happening with your question, I'm going to pretend the variable is for drinking a potion of power. I'll name the potion POWPOT because I want to. I might set the min value to 0, the max value to 1, and the starting value to 0.
Then, on page 9 (or wherever you want to do this), one of my options is going to be to drink the potion and go on. I'm guessing, based on your question, that another option on that page is to just go on without drinking the potion. So that option will be just a normal link to page 10. The option to drink the potion and go on will also be a simple link to page 10. That way whether they drink the potion or not, they're going to page 10. But, on the link for the drink a potion option, I'm going to
set a variable. To do that, I'm going to the link to Page 10 and I'll click on the percent sign to modify variables. That will open a new window and window will have a list of my variables. I'll go to my POWPOT row, select addition for my change type, and put a 1 in the change box. Then I'll save those changes and close.
Now that the variable's set, I can go to the other page. I don't know if that's page 12 or 17 in your example, but that doesn't matter. At some point, I want to go to a different page. In my example, I'm going to have an option to attempt to crash through a door. I want the reader to have one option, attempt to crash through the door, that will result in page 403 if they have already taken the potion, but page 101 if they have not taken the potion (and there might be another option or four, like turn the handle and open the door, what I'm doing here won't care if you have other options). To do that, I'll create two links that appear identical, but go to different pages. Just create the two links to page 403 and page 101. For the text of each link I'll put the same text, "Attempt to crash through the door." But on each link, I'll add a link restriction by clicking on the little square inside the circle to the left of the link text. That will open a new window. In the top menu of that new window, I'll click on the "Variable restrictions" option. That will show me a list of my variables, where I'll see the POWPOT row. For the page 403 link, I'll select = from type and put 1 in the value. For the 101 link, I'll select = from the type and put 0 in the value. Once that's saved, it should work as I have described here.
You can do this with any number of variables and links in any number of places in your story at any time.
Have fun!
