diff --git a/services/core/java/com/android/server/am/ActiveServices.java b/services/core/java/com/android/server/am/ActiveServices.java
index a8046086a11de..ee18ed5667255 100644
--- a/services/core/java/com/android/server/am/ActiveServices.java
+++ b/services/core/java/com/android/server/am/ActiveServices.java
@@ -7429,14 +7429,17 @@ public final class ActiveServices {
 
         final int uidState = mAm.getUidStateLocked(callingUid);
         if (ret == REASON_DENIED) {
-            // Is the calling UID at PROCESS_STATE_TOP or above?
+            // Allow FGS while-in-use if the caller's process state is PROCESS_STATE_PERSISTENT,
+            // PROCESS_STATE_PERSISTENT_UI or PROCESS_STATE_TOP.
             if (uidState <= PROCESS_STATE_TOP) {
                 ret = getReasonCodeFromProcState(uidState);
             }
         }
 
         if (ret == REASON_DENIED) {
-            // Does the calling UID have any visible activity?
+            // Allow FGS while-in-use if the caller has visible activity.
+            // Here we directly check ActivityTaskManagerService, instead of checking
+            // PendingStartActivityUids in ActivityManagerService, which gives the same result.
             final boolean isCallingUidVisible = mAm.mAtmInternal.isUidForeground(callingUid);
             if (isCallingUidVisible) {
                 ret = REASON_UID_VISIBLE;
@@ -7444,7 +7447,8 @@ public final class ActiveServices {
         }
 
         if (ret == REASON_DENIED) {
-            // Is the allow activity background start flag on?
+            // Allow FGS while-in-use if the background activity start flag is on. Because
+            // activity start can lead to FGS start in TOP state and obtain while-in-use.
             if (backgroundStartPrivileges.allowsBackgroundActivityStarts()) {
                 ret = REASON_START_ACTIVITY_FLAG;
             }
@@ -7453,6 +7457,7 @@ public final class ActiveServices {
         if (ret == REASON_DENIED) {
             boolean isCallerSystem = false;
             final int callingAppId = UserHandle.getAppId(callingUid);
+            // Allow FGS while-in-use for a list of special UIDs.
             switch (callingAppId) {
                 case ROOT_UID:
                 case SYSTEM_UID:
@@ -7471,6 +7476,10 @@ public final class ActiveServices {
         }
 
         if (ret == REASON_DENIED) {
+            // Allow FGS while-in-use if the WindowManager allows background activity start.
+            // This is mainly to get the 10 seconds grace period if any activity in the caller has
+            // either started or finished very recently. The binding flag
+            // BIND_ALLOW_BACKGROUND_ACTIVITY_STARTS is also allowed by the check here.
             final Integer allowedType = mAm.mProcessList.searchEachLruProcessesLOSP(false, pr -> {
                 if (pr.uid == callingUid) {
                     if (pr.getWindowProcessController().areBackgroundFgsStartsAllowed()) {
@@ -7485,6 +7494,12 @@ public final class ActiveServices {
         }
 
         if (ret == REASON_DENIED) {
+            // Allow FGS while-in-use if the caller UID is in ActivityManagerService's
+            // mFgsWhileInUseTempAllowList. This is a temp allowlist to allow FGS while-in-use. It
+            // is used when MediaSessionService's bluetooth button or play/resume/stop commands are
+            // issued. The typical temp allowlist duration is 10 seconds.
+            // This temp allowlist mechanism can also be called by other system_server internal
+            // components such as Telephone/VOIP if they want to start a FGS and get while-in-use.
             if (mAm.mInternal.isTempAllowlistedForFgsWhileInUse(callingUid)) {
                 return REASON_TEMP_ALLOWED_WHILE_IN_USE;
             }
@@ -7492,6 +7507,8 @@ public final class ActiveServices {
 
         if (ret == REASON_DENIED) {
             if (targetProcess != null) {
+                // Allow FGS while-in-use if the caller of the instrumentation has
+                // START_ACTIVITIES_FROM_BACKGROUND permission.
                 ActiveInstrumentation instr = targetProcess.getActiveInstrumentation();
                 if (instr != null && instr.mHasBackgroundActivityStartsPermission) {
                     ret = REASON_INSTR_BACKGROUND_ACTIVITY_PERMISSION;
@@ -7500,6 +7517,9 @@ public final class ActiveServices {
         }
 
         if (ret == REASON_DENIED) {
+            // Allow FGS while-in-use if the caller has START_ACTIVITIES_FROM_BACKGROUND
+            // permission, because starting an activity can lead to starting FGS from the TOP state
+            // and obtain while-in-use.
             if (mAm.checkPermission(START_ACTIVITIES_FROM_BACKGROUND, callingPid, callingUid)
                     == PERMISSION_GRANTED) {
                 ret = REASON_BACKGROUND_ACTIVITY_PERMISSION;
@@ -7507,6 +7527,8 @@ public final class ActiveServices {
         }
 
         if (ret == REASON_DENIED) {
+            // Allow FGS while-in-use if the caller is in the while-in-use allowlist. Right now
+            // AttentionService and SystemCaptionsService packageName are in this allowlist.
             if (verifyPackage(callingPackage, callingUid)) {
                 final boolean isAllowedPackage =
                         mAllowListWhileInUsePermissionInFgs.contains(callingPackage);
@@ -7521,7 +7543,7 @@ public final class ActiveServices {
         }
 
         if (ret == REASON_DENIED) {
-            // Is the calling UID a device owner app?
+            // Allow FGS while-in-use if the caller is the device owner.
             final boolean isDeviceOwner = mAm.mInternal.isDeviceOwner(callingUid);
             if (isDeviceOwner) {
                 ret = REASON_DEVICE_OWNER;