What could "BYTE ORDER = I" mean?

You're right that the usual suspects are LE (Little Endian) and BE (Big Endian). But "I"? That’s not a standard label — so here are a few possibilities that might explain it:

• "I" as in Intel-style: Some systems or file formats use "I" to indicate Intel byte order, which is just another way of saying Little Endian. Since Intel processors are Little Endian by default, "I" might be shorthand for that convention.

• "I" for Independent or Interchangeable: In rare cases, "I" could imply that the file is endian-neutral — meaning the byte order doesn’t matter for certain parts of the data (like ASCII text). But this is speculative unless the file format explicitly says so.

• Proprietary or internal marker: Since this is from a Magellan Roadmate map file, "I" might be a custom marker used by their firmware or software. Embedded systems sometimes use non-standard indicators for internal parsing.