The J Med Internet Res has an emphasis on Internet research but also publishes - more broadly - anything that has to do with innovations using digital technology (eHealth). As the flagship journal, acceptance standards are higher. Reviewers should suggest a transfer to another JMIR journal if the paper is in principle publishable but not strong enough for the top journal in the field (the paper should be cited at least 5-10 times in year 3 after publication).
Note that for all other journals other than J Med Internet Res, the "JMIR" in the title is the publisher brand (What does "JMIR" in the title of your journals stand for? ) - it does NOT imply that all JMIR journals deal with the "Internet" exclusively.
JMIR mHealth and uHealth focuses on mobile apps and ubiquitous health e.g. domotics, sensors etc.
JMIR Research Protocols publishes research protocols and proposals only - i.e. these papers usually have no substantial results to report yet, and this should obviously not be held against the authors. Publishing protocols before data collection increases accountability.
While no substantive results are expected, a results section is still necessary to report the current status and expected timeline, even if it is just 1-2 sentences (see What should I put into the results section of a protocol or proposal?).
Note that JMIR Research Protocols publishes protocols from all areas of biomedical research, not just Internet or digital health research.
Please read the scope statement of other JMIR journals on the journal homepage.