Pressing Alt-R in the record list brings up a dialog that allows you to restrict which records will appear in the record list. It will also affect which records are modified by a Modify All operation and which records are exported if the "Selected records only" option is used when exporting.
The Restrict Records dialog allows for two different methods of restricting records of which you can use either or both. The first is a minimum and / or maximum key value for the current index as selected by Alt-S. A From and To field is given for each segment in the current index. Note that it doesn't make sense to leave gaps in the segment list as each segment is less significant than the one above it. A key value should be filled in from the first segment downwards. If you want to restrict by a field that isn't the first segment of any index you'll need to use a filter expression.
The second method uses a filter expression that evaluates to a boolean value (true or false). A record is included only if the expression evaluates to true for that record. This method can be much slower than the first as every potential record must be retrieved and tested so you should always try to restrict by key value if possible before resorting to a filter expression.