Hi J J,
Summary Fields Do Not appear under 'Select Expert', however they are accessible under 'Group Expert'.
Parameter fields on the other hand are visible under both of them.
P.S: You cannot use a Summary Field in the Select Expert/Record Selection Formula because summation happens at a later stage in the report design whereas selection happens at the initial stages. Open CR, hit the F1 button and search for " Multi-pass reporting flow chart" to understand more on the execution engine.
-Abhilash