Document enable_autocheckpoint and its distinction from emergency checkpointing.

docs/guides/checkpointing_solutions/emergency_checkpointing.md

@@ -113,6 +113,16 @@ MaxText provides a set of configuration flags to control checkpointing options.
 | `local_checkpoint_period`              | The interval, in training steps, for how often a **local checkpoint** is saved. This should be set to a much smaller value than `checkpoint_period` for frequent, low-overhead saves.                                                                                                                    | `integer` | `0`     |
 | `checkpoint_period`                    | The interval, in training steps, for how often a checkpoint is saved to **persistent storage**.                                                                                                                                                                                                          | `integer` | `10000` |
 | `enable_single_replica_ckpt_restoring` | If `True`, one replica reads the checkpoint from storage and then broadcasts it to all other replicas. This can significantly speed up restoration on multi-host systems by reducing redundant reads from storage.                                                                                       | `boolean` | `False` |
+| `enable_autocheckpoint`                | If `True`, enables saving a checkpoint when a preemption signal (SIGTERM) is received. This is a reactive mechanism that saves to persistent storage.                                                                                                                                                    | `boolean` | `False` |
+
+### Autocheckpoint vs. Emergency Checkpointing
+
+While both features aim to protect against progress loss, they operate differently:
+
+- **Autocheckpoint (`enable_autocheckpoint`)**: A **reactive** mechanism. When the infrastructure sends a `SIGTERM` signal (indicating imminent preemption or maintenance), MaxText immediately attempts to save a checkpoint to persistent storage (GCS). It is best for handling planned maintenance or preemptions where a short grace period is provided.
+- **Emergency Checkpointing (`enable_emergency_checkpoint`)**: A **proactive** mechanism. It saves checkpoints very frequently to local, high-speed storage (ramdisk). If a failure occurs *without* warning, the job can recover from the most recent local checkpoint. It is best for handling sudden hardware failures.
+
+For maximum reliability, both features can be enabled simultaneously.
 
 ## Workload creation using XPK
 

docs/reference/core_concepts/checkpoints.md

@@ -95,6 +95,7 @@ MaxText automatically saves checkpoints periodically during a training run. Thes
 Furthermore, MaxText supports emergency checkpointing, which saves a local copy of the checkpoint that can be restored quickly after an interruption.
 
 - `enable_emergency_checkpoint`: A boolean to enable or disable this feature.
+- `enable_autocheckpoint`: A boolean to enable or disable saving a checkpoint when a preemption signal (SIGTERM) is received.
 - `local_checkpoint_directory`: The local path for storing emergency checkpoints.
 - `local_checkpoint_period`: The interval, in training steps, for saving local checkpoints.