ENH: Note that data types are the directories containing data files

Author: effigies

src/02-common-principles.md

@@ -33,12 +33,19 @@ misunderstanding we clarify them here.
     a brain scanning instrument was acquiring data according to particular
     scanning sequence/protocol.
 
-1.  **Data type** - a functional group of different types of data. In BIDS we
-    define eight data types: `func` (task based and resting state functional MRI),
-    `dwi` (diffusion weighted imaging), `fmap` (field inhomogeneity mapping data
-    such as field maps), `anat` (structural imaging such as T1, T2, etc.), `meg`
-    (magnetoencephalography), `eeg` (electroencephalography), `ieeg` (intracranial
-    electroencephalography), `beh` (behavioral).
+1.  **Data type** - a functional group of different types of data.
+    BIDS defines eight data types:
+    `func` (task based and resting state functional MRI),
+    `dwi` (diffusion weighted imaging),
+    `fmap` (field inhomogeneity mapping data such as field maps),
+    `anat` (structural imaging such as T1, T2, etc.),
+    `meg` (magnetoencephalography),
+    `eeg` (electroencephalography),
+    `ieeg` (intracranial electroencephalography),
+    `beh` (behavioral).
+    Data files are contained in a directory named for the data type.
+    In raw datasets, the data type directory is nested inside subject and
+    (optionally) session directories.
 
 1.  **Task** - a set of structured activities performed by the participant.
     Tasks are usually accompanied by stimuli and responses, and can greatly vary