clarify that label of _space-<label> must be from appendix VIII

related to https://github.com/bids-standard/bids-validator/issues/743

src/04-modality-specific-files/04-intracranial-electroencephalography.md

@@ -288,9 +288,8 @@ that coordinates are expected in cartesian coordinates according to the
 `*_coordsystem.json` file MUST be specified as well.
 
 The optional [`space-<label>`](../99-appendices/09-entities.md#space) entity (`*[_space-<label>]_electrodes.tsv`) can be used to
-indicate the way in which electrode positions are interpreted. The space label
-needs to be taken from the list in
-[Appendix VIII](../99-appendices/08-coordinate-systems.md)
+indicate the way in which electrode positions are interpreted.
+The space `<label>` MUST be taken from the list in [Appendix VIII](../99-appendices/08-coordinate-systems.md)
 
 For examples:
 

src/schema/entities.yaml

@@ -197,7 +197,8 @@ space:
     the way in which electrode positions are interpreted
     (for EEG/MEG/iEEG data) or
     the spatial reference to which a file has been aligned (for MRI data).
-    The space label needs to be taken from the list in Appendix VIII.
+    The space label MUST be taken from the list in
+    [Appendix VIII](../99-appendices/08-coordinate-systems.md)
 
     For EEG/MEG/iEEG data, this entity can be applied to raw data, but
     for other data types, it is restricted to derivative data.