Max() and tag value

I’m a beginner with influxDB and I searched on the forum to find the answer but I didn’t find it. I would like a simple thing: get a max value of a measurement.

I tried the following request but it didn’t work (no result).
SELECT MAX(“ON”),“T1” FROM “test”.“autogen”.“Data”
“ON” is tag and “T1” is a field.

When i tried the following request, it works well:
SELECT MAX(“T1”),“ON” FROM “test”.“autogen”.“Data”

Maybe I didn’t understand the difference between tag and field. For me, if i need to make frequently query to search a value (WHERE clause), it should be a tag. I consider the MAX() function as a search so i put “ON” in the tag category.
Is it an error ?
Why the first query return nothing ? Is it possible to have the max() value of a tag ?
Thank you for your help.