[FIX] Clarify MEG empty-room recommendations

src/04-modality-specific-files/02-magnetoencephalography.md

@@ -588,26 +588,26 @@ has to be updated, then for MEG it could be considered to be a new session.
 
 ## Empty-room MEG recordings
 
-Empty-room MEG recordings capture the environmental and recording system's
-noise.
-In the context of BIDS it is RECOMMENDED to perform an empty-room recording for
-each experimental session.
-It is RECOMMENDED to store the empty-room recording inside a subject directory
-named `sub-emptyroom`.
-The label for the [`task-<label>`](../99-appendices/09-entities.md#task) entity in the empty-room recording SHOULD be
-set to `noise`.
-If a [`session-<label>`](../99-appendices/09-entities.md#ses) entity is present, its label SHOULD be the date of the
-empty-room recording in the format `YYYYMMDD`, that is `ses-YYYYMMDD`.
-The `scans.tsv` file containing the date and time of the acquisition SHOULD
-also be included.
-The rationale is that this naming scheme will allow users to easily retrieve the
-empty-room recording that best matches a particular experimental session, based
-on date and time of the recording.
-It should be possible to query empty-room recordings just like usual subject
-recordings, hence all metadata sidecar files (such as the `channels.tsv`) file
-SHOULD be present as well.
+Empty-room MEG recordings capture the environmental and recording system's noise.
 
-Example:
+It is RECOMMENDED to explicitly specify which empty-room recording should be used with which experimental run(s) or session(s). This can be done via the [`AssociatedEmptyRoom`](../99-appendices/14-glossary.md#associatedemptyroom-metadata) field in the `*_meg.json` sidecar files.
+
+Empty-room recordings may be collected once per day, where a single empty-room recording may be shared between multiple subjects and/or sessions (see example 1).
+Empty-room recordings can also be collected for each individual experimental session (see example 2).
+
+In the case of empty-room recordings being associated with multiple subjects and/or sessions, it is RECOMMENDED to store the empty-room recording inside a subject directory named `sub-emptyroom`.
+If a [`session-<label>`](../99-appendices/09-entities.md#ses) entity is present, its label SHOULD be the date of the empty-room recording in the format `YYYYMMDD`, that is `ses-YYYYMMDD`.
+The `scans.tsv` file containing the date and time of the acquisition SHOULD also be included.
+The rationale is that this naming scheme will allow users to easily retrieve the empty-room recording that best matches a particular experimental session, based on date and time of the recording.
+It should be possible to query empty-room recordings just like usual subject recordings, hence all metadata sidecar files (such as the `channels.tsv`) file SHOULD be present as well.
+
+In the case of empty-room recordings being collected for the individual experimental session, it is recommended to store the empty-room recording along with that subject and session.
+
+In either case, the label for the [`task-<label>`](../99-appendices/09-entities.md#task) entity in the empty-room recording SHOULD be set to `noise`.
+
+Example 1:
+
+One empty-room recording per day, applying to all subjects for that day.
 
 <!-- This block generates a file tree.
 A guide for using macros can be found at
@@ -630,4 +630,30 @@ A guide for using macros can be found at
    }
 ) }}
 
+Example 2:
+
+One recording per session, stored within the session folder.
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "sub-control01": {
+      "ses-01": {
+         "sub-01_ses-01_scans.tsv": "",
+         "meg": {
+            "sub-control01_ses-01_task-rest_meg.ds": "",
+            "sub-control01_ses-01_task-rest_meg.json": "",
+            "sub-control01_ses-01_task-rest_channels.tsv": "",
+            "sub-control01_ses-01_task-noise_meg.ds": "",
+            "sub-control01_ses-01_task-noise_meg.json": "",
+            "sub-control01_ses-01_task-noise_channels.tsv": "",
+            }
+         }
+      },
+   }
+) }}
+
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