Added comment explaining changes from previous commit.

Author: Nikhil Chandra

neo/rawio/plexon2rawio/plexon2rawio.py

@@ -231,6 +231,13 @@ def _parse_header(self):
             if not (schannel_info.m_ChannelEnabled and schannel_info.m_ChannelRecordingEnabled):
                 continue
 
+            # In a PL2 spike channel header, the field "m_NumberOfUnits" denotes the number
+            # of units to which spikes detected on that channel have been assigned. It does
+            # not account for unsorted spikes, i.e., spikes that have not been assigned to 
+            # a unit. It is Plexon's convention to assign unsorted spikes to channel_unit_id=0, 
+            # and sorted spikes to unit_id's 1, 2, 3...etc. Therefore, for a given value of 
+            # m_NumberOfUnits, there are m_NumberOfUnits+1 channel_unit_ids to consider - 1 
+            # unsorted channel_unit_id (0) + the m_NumberOfUnits sorted channel_unit_ids. 
             for channel_unit_id in range(schannel_info.m_NumberOfUnits+1):
                 unit_name = f"{schannel_info.m_Name.decode()}.{channel_unit_id}"
                 unit_id = f"unit{schannel_info.m_Channel}.{channel_unit_id}"