Retention frame (10, 20) having separate plug-in modules (40, 40', 40") for a system plug connector having at least one such retention frame (10, 20) and an additional retention frame and a connector housing, wherein the plug-in modules (40, 40', 40") contain electrical contacts and are retained by the retention frames, wherein the retention frame (10, 20) has: a rectangular duct (11, 21) with devices for retaining the plug-in modules (40, 40', 40"), a plug insert which is arranged in the rectangular duct (11, 21) and within which the plug-in modules (40, 40', 40") can be inserted into the retention frame (10, 20), wherein the plug-in modules (40, 40', 40") can be arranged so as to be able to be secured in the rectangular duct (11, 12), characterised in that the retention frame further has slide rails (14, 14', 24, 24') which are arranged opposite and which are formed in the rectangular duct (11, 21), wherein the retention frame (10, 20) is configured in such a manner that the plug-in modules (40, 40', 40") can be inserted at both sides, from an insertion side and from a connection side, perpendicularly to the retention frame into the rectangular duct (11, 12), wherein the retention frame (10, 20) is further configured in such a manner that a plug-in module (40, 40') which is inserted into the plug insert and which has in each case at opposing sides two transverse ribs and a groove which is arranged therebetween, can be displaced within the retention frame (10, 20) transversely relative to the insertion direction laterally as far as a stop, wherein the slide rails (14, 14', 24, 24') engage in each case in the grooves, wherein the plug insert is constructed as a separate, U-shaped and electrically conductive portion and is in abutment with three sides within the rectangular duct (11, 21) of the retention frame (10, 20), the plug insert has in the side frames thereof, which are orientated in a U-shaped manner, on the upper and lower edge two free-standing tongues which are arranged in a parallel manner and which are directed into the centre of the side frame and whose spaced-apart tongue ends are bent into the rectangular duct (11, 21). a rectangular duct (11, 21) with devices for retaining the plug-in modules (40, 40', 40"), a plug insert which is arranged in the rectangular duct (11, 21) and within which the plug-in modules (40, 40', 40") can be inserted into the retention frame (10, 20), wherein the plug-in modules (40, 40', 40") can be arranged so as to be able to be secured in the rectangular duct (11, 12), characterised in that the retention frame further has slide rails (14, 14', 24, 24') which are arranged opposite and which are formed in the rectangular duct (11, 21), wherein the retention frame (10, 20) is configured in such a manner that the plug-in modules (40, 40', 40") can be inserted at both sides, from an insertion side and from a connection side, perpendicularly to the retention frame into the rectangular duct (11, 12), wherein the retention frame (10, 20) is further configured in such a manner that a plug-in module (40, 40') which is inserted into the plug insert and which has in each case at opposing sides two transverse ribs and a groove which is arranged therebetween, can be displaced within the retention frame (10, 20) transversely relative to the insertion direction laterally as far as a stop, wherein the slide rails (14, 14', 24, 24') engage in each case in the grooves, wherein the plug insert is constructed as a separate, U-shaped and electrically conductive portion and is in abutment with three sides within the rectangular duct (11, 21) of the retention frame (10, 20), the plug insert has in the side frames thereof, which are orientated in a U-shaped manner, on the upper and lower edge two free-standing tongues which are arranged in a parallel manner and which are directed into the centre of the side frame and whose spaced-apart tongue ends are bent into the rectangular duct (11, 21).