Here's where to look in the Narrator's Guide
Here's where to look in the Narrator's Guide, on page 115, under the heading "System Daamge Tracks. Second paragraph in the section:
"Every system recorded on the starship profile has a reliability rating, ranging from A to E. Systems with a Class A reliability lack backup and redundant systems and are more easily damaged in battle. Systems with Class E reliability can sustain more damage while continuing to function. (For more information on reliability ratings, see page 137). This reliability rating establishes an individual damage track for each system."
This is just above the example System Damage Tracks graphic, and right in with all the rules on damagong ships and systems, and even includes a cross-reference to the Reliability rules on page 137. It might have been a tad clearer, but it's hardly opaque, and it's certainly in a rational place in the book.