After the initial TreeShare upload, there are not really any options about what to share. When you go into TreeShare’s dialog to do a “manual sync” (it’s not a true automatic sync), you will be shown the differences between RM and Ancestry on a person by person basis. You can see these differences either for every person, or only for recently changed persons. After an initial TreeShare upload, there is nobody on the recently changes list. But as you make changes in either RM or Ancestry, people are added to the recently changed list.
Persons get on the recently changed list based on their change date in RM and Ancestry, not on whether their data is actually different. In fact, immediately after an initial TreeShare upload, you are likely to have people whose data is actually different but who are not on the recently changed list.
After people get on the recently changed list (or even if they are not on the recently changed list and you notice some differences), you as the user look at the data and decide if the data is actually different or not. For each item that is different at that point, you make a decision about what to share and what not to share, and in which direction to share - RM to Ancestry or Ancestry to RM.
Also, suppose on the initial TreeShare upload that you choose some categories of information not to share. People with those categories of information will therefore be different between RM and Ancestry. But they will not be shown on the list of people with recent changes. If you look at such people with TreeShare anyway, you will see the differences and you will still be offered the opportunity make the people the same. The initial upload options are not remembered in any way. Instead, you have total and manual control of the changes after the initial upload.
There are a number of discussions about these kinds of issues. Here is one such discussion. There used to be a very good summary somewhere that describes the differences in data model between RM and Ancestry which in turn describes which data will not transfer cleanly between the two. But at the present time, I cannot find the link to that summary.