The closing part of 14.17.2 says: Note that if the second partition's type is chosen ^^^^^^ appropriately, it is possible to have OpenBSD access both partitions on the device. So, a Windows user could populate the FAT32 partition with MP3 files which could be played when they booted from the OpenBSD partition.
I believe that calling it "the other partition" would be better wording, especially in the context of emphasizing how some other OS's might need their partition to be the _first_.