Hi fernand-7,
I'm not sure if BP has a guide on this or not, but the Filter input seems to be equivalent to a SQL WHERE clause. For example, if you're working with comparing dates, you need to put # around the dates such as ""[dateField]>#9/27/2018#"". And if you need to use a wildcard, you can use LIKE such as (and no I'm not Miss SC): ""[textField] LIKE '%some text%'"".
w3schools is always a good place to start: https://www.w3schools.com/sql/sql_where.asp
Respectfully,
Dave
Dave Morris, 3Ci at Southern Company