In addition to specifying infield: queries in-line with the main search, they may also be given with the infq form variable. This variable may be specified multiple times, with one query per value. (For example, the "With this field" box on the Advanced Search form uses the infq variable to set its additional query.) Since these infq queries are given separately from the main search query, there is no need for the infield: prefix, and it must not be given; otherwise the syntax is identical to infield: queries.
For example, the main search query (i.e. set in query variable):
Acme widgets infield:Gauge:24..32
would search the standard text fields for Acme widgets, and search the Gauge Parametric field for values from 24 to 32 inclusive. This same combined query could also be given as:
Acme widgets
in the query variable, with:
Gauge:24..32
in the infq variable.