When using Excel files as Data Sources you can only call functions within the Query that are VBA functions.
An example is when querying an excel file as the source of a call list import template. If you only wanted to upload the entries that have a value in the spreadsheets "CallDate" column of Todays Date you could use the below:
SELECT * FROM [WORKSHEET1$] WHERE CallDate = Date()
The reason for this is that CallScripter uses OLE DB to pass the query down to ADO that then queries the excel spreadsheet. ADO executes the query within VBA and so only functions supported by VBA are allowed.
OLE DB = driver that CallScripter talks via to non SQL Databases.
ADO = Microsoft Office Access toolkit
VBA = Coding language used with Micorsoft Office (Visual Basic)