The MobileApps service will be tightly integrated with RESTBase (for storage/caching/invalidation), which deals with exact revision IDs and/or TIDs for pages. On the other hand, the current MobileApps route accepts only the title. It needs to be able to accept precise revision IDs and TIDs, so it can request a specific version (and render) of a page.
It's a time ID, i.e. the time stamp indicating the time a revision has been rendered. Each revision can be rendered multiple times with different content without changing the actual underlying wikitext. The most notable examples of that are dynamic templates which change a page's rendered HTML output without changing the actual page source. Since RESTBase aims at providing historical records for pages, a revision might have multiple renders - and we need to know exactly which one are we dealing with.
How would I be able to specify that action=mobileview use a specific revision ID or TID?
Hm, interesting. According to its help section, there indeed is no way of providing a specific revision ID. Instead, only the latest seems to be supported (or, assumed, rather). So, apparently, we would need to put support for that in the MobileView API.