Hey prospectheights, that's a great question. Since I have not used the app for this purpose myself, I'm going to reach out to our team to get that clarified. Stay tuned!
I also wanted to give you a warm welcome to Insiders! This forum is a great place to share experiences, perspectives, photos, and ask questions to a group of experienced travelers. Being new, feel free to check out our Getting Started Guide for tips on how to enhance your Insiders experience. Always feel free to reach out to myself (CariJ), lindseyh, or joshm with any questions or concerns about anything Marriott Rewards or the Insiders site. We are the community managers of the forum and are here to help!
Hey prospectheights, our team confirmed that any request made (via app, website, or phone call) is not a guarantee. Request are not a guarantee until check-in, because it is based on availability at that time.
But, CariJ - I am guessing that the question from prospectheights is really this: does using mobile check-in make it more likely, less likely, or no difference, that special requests will be honored. Personally, for something very specific like adjoining rooms with a connecting door, I would probably call on the morning of check-in to see if those rooms could be set aside. But the more general question is, for possible upgrades, or possible good room locations in a property where it makes a difference, do I improve my chances by using mobile check-in early in the day, or am I better off checking in in person so I can make the request of a live human being?