diff --git a/docs/REPOS.md b/docs/REPOS.md
index 1eb2bdb..82087b4 100644
--- a/docs/REPOS.md
+++ b/docs/REPOS.md
@@ -156,6 +156,10 @@ Two Orin Nanos, and they have different jobs:
 | Development Orin (`mikkeli-orin-nano-2`) | Build, CI runner, bench testing. Always powered, on the LAN |
 | Car Orin (`mikkeli-orin-nano`) | Deployment target only |
 
+The SIM7600 LTE/GNSS hat lives in the car unit only — its custom enclosure doesn't
+move between units. Anything touching the modem or gpsd is tested on the car unit;
+on the development unit, `gnss` runs against its own mock input (decision 19).
+
 ⚠ **The guarantee is version parity, not architecture.** An aarch64 binary built
 on a newer userland still fails to load on an older one. A JetPack or Ubuntu
 upgrade applied to one unit and not the other reintroduces the problem, silently