(ssget "_:L") - stops users from selecting objects on locked layers
(ssget "_:P") - rejects selection of viewports
(ssget "_:R") - allow selection in long transactions
(ssget "_:U") - enable subentity selection with Ctrl
(ssget "_:V") - force subentity selection
(ssget "_+.") - allows window selection even in the single selection mode - e.g. (ssget "_+.:E:S") (may not work in some localized versions of AutoCAD)