You can download the main MIDI specification as a PDF file from the following page on this site: Specs -> The MIDI 1.0 Specification ->
The Complete MIDI 1.0 Detailed Specification. (You have to log in with your account to download the PDF file.)
Bottom of PDF page 36:
There are three types of System messages: Common, Real-Time, and Exclusive.
PDF page 37: [...]
EXCLUSIVE: Exclusive messages can contain any number of Data bytes, and can be terminated either by an End of Exclusive (EOX) or any other Status byte (except Real Time messages). An EOX should always be sent at the end of a System Exclusive message.
I guess that means you should normally end a System Exclusive message with an F7. But if you happen to use a 90 after an F0 and before an F7, the System Exclusive message is terminated, so that the 90 is properly processed as a Note On message.