Information on the System Variables and descriptions can be found in the TIBCO Scribe Insight help guide by following the link below.

https://help.scribesoft.com/scribeinsight/en/index.htm#Subsystems/Insight/configsource/sqlqueries/using_system_variables.htm?

When using System Variables that end in "DTS" (e.g. :LastRunDateTimeDTS, :LastRunDateTimeUTCDTS, :PrevRunDateTimeDTS, :PrevRunDateTimeUTCDTS) you must put the full file path of the DTS you are using within double quotes, within parentheses. This is required as the system variable queries the SCRIBEINTERNAL database and needs to know the exact DTS to use with the query.  

The following is an example of the proper way to input the variable with the full path to the DTS file. This is an example you might use in the Source Query window.

Select * from address where last_mdfy_date >= :LastRunDateTimeDTS("D:\Scribe\Collaborations\Default\AccountUpdateCRM.dts")

When using System Variables that ends in "DTS", the full path to the DTS file needs to be provided to the variable for it to work correctly.