Using-pepcli.md

@@ -119,6 +119,8 @@ pepcli ama column create <column name>
 pepcli ama column remove <column name>
 ```
 
+Because of technical limitations, PEP column names may contain only [printable ASCII characters](https://en.wikipedia.org/wiki/ASCII#Printable_characters). Additional restrictions apply to the names of columns into which Castor data are imported @@@ link and/or describe @@@.
+
 Note that column removal will not discard data present in those columns; it will merely make the column's contents inaccessible. Therefore:
 
 - when users retrieve data from an earlier moment in time, those data may include columns that have since been removed.
@@ -144,6 +146,8 @@ pepcli ama columnGroup create <column group name>
 pepcli ama columnGroup remove <column group name>
 ```
 
+Because of technical limitations, column group names may contain only [printable ASCII characters](https://en.wikipedia.org/wiki/ASCII#Printable_characters). Note that [some column groups](https://gitlab.pep.cs.ru.nl/pep-public/user-docs/-/wikis/Data-structure#grouping) are predefined and/or managed by PEP itself.
+
 Once a column group has been created, use the [`pepcli ama column`](#ama-column) subcommand to determine which columns are included in the group. `Access Administrator` can [grant access to column groups](Access-control#access-rules) using the `pepcli ama cgar` subcommand.
 
 ### `ama query`