Clarify WildFIRE-FM validity mask channels

models/wildfire_fm/input_channels.json

@@ -53,7 +53,9 @@
     "FIRMS active-fire detections are used to derive the occupancy target, not as an input channel.",
     "WFIGS and MTBS are supporting event-level resources and are not part of the pretrained occupancy input.",
     "The regional cache contains zero firewx feature channels and one firewx_valid channel, so the released input has 16 channels total.",
-    "CAPE is selected from the HRRR surface instantaneous cfgrib group: typeOfLevel=surface and stepType=instant. It is not the most-unstable or layer CAPE channel."
+    "CAPE is selected from the HRRR surface instantaneous cfgrib group: typeOfLevel=surface and stepType=instant. It is not the most-unstable or layer CAPE channel.",
+    "firewx_valid is a cache-level dynamic input presence mask. In the released California regional cache it is 1.0 for every grid cell and is not a per-variable HRRR missing-data mask.",
+    "static_valid is the fraction of the four static layers that are valid at a grid cell after reprojection. It can take values 0.0, 0.25, 0.5, 0.75, or 1.0."
   ],
   "channels": [
     {
@@ -200,17 +202,19 @@
       "index": 10,
       "name": "firewx_valid",
       "source": "cache validity channel",
-      "units": "mask",
+      "units": "binary mask",
       "normalization": "none",
-      "role": "dynamic/input validity mask for this regional cache"
+      "values": [1.0],
+      "role": "cache-level dynamic input presence mask for this regional cache; not a per-variable HRRR missing-data mask"
     },
     {
       "index": 11,
       "name": "static_valid",
       "source": "static reprojection mask",
-      "units": "mask",
+      "units": "fraction",
       "normalization": "none",
-      "role": "fraction of static layers valid at the grid cell"
+      "values": [0.0, 0.25, 0.5, 0.75, 1.0],
+      "role": "fraction of static layers valid at the grid cell after reprojection"
     },
     {
       "index": 12,