Wayne - The Category data type is a subset of the Text data type applied where the number of unique values in a field are few enough to be useful in visualisation and filtering, for example, you can split Pie Views by a Category field like [Day of Week], because there are only 7 possible values, making a nice pie view with seven wedges.

If a Text field has more than 100 unique values, it will be typed Category in OMniscope files by default. You can 'force' a field with more than 100 unique values to be typed Category, but there will be a performance penalty. On data refresh, since the total number of unique values is unpredictable, there is a protective default setting to re-type too numerous incoming values from Category to Text (this will also remove the field, which is no longer typed Category, from various menus like the drop-down split by options in the Pie View.

If you ALWAYS want to 'force' a text field to be typed Category, (regardless of the number of future incoming values and the performance implications) you should change the following setting for the particular file:

Settings {this file} Advanced file settings > (untick) Auto-convert text fields