The top level of a frame structure suitable for a warehousing or manufacturing environment for supporting robotic load handling devices, the top level comprising a first set of parallel rails extending in an X direction and a second set of parallel rails extending in a Y direction perpendicular to the X direction, the X and Y directions being in a horizontal plane on the top level of the frame structure; wherein the first and second sets of rails (22a, 22b) form a uniform grid pattern across the top of the stacks comprising an array of openings, each opening being a rectangular opening and having a width in the X direction and a length in the Y direction, wherein each rail in the second set of rails comprises a longitudinally extending dividing structure, centrally dividing the respective rail into two adjacent tracks, each track being engageable by a wheel of a robot service device, characterised in that: each opening is surrounded along its width and length, by a raised lip provided by the rails (22a, 22b); wherein each rail in the first set of rails comprises a longitudinally extending dividing structure, centrally dividing the respective rail into two adjacent tracks, each track being engageable by a wheel of a robot service device, and wherein the dividing structure of each rail of the first set of rails comprises a series of raised dividers, each raised divider extending along, and parallel to, the full width of an adjacent opening, and wherein the dividing structure of each rail of the second set of rails comprises a series of raised dividers, each raised divider extending along, and parallel to, the full length of an adjacent opening; and wherein each raised divider of the dividing structure of each rail of the first set of rails extends no further than the width of the respective adjacent opening, and each raised divider of the dividing structure of each rail of the second set of rails extends no further than the length of the respective adjacent opening. wherein the first and second sets of rails (22a, 22b) form a uniform grid pattern across the top of the stacks comprising an array of openings, each opening being a rectangular opening and having a width in the X direction and a length in the Y direction, wherein each rail in the second set of rails comprises a longitudinally extending dividing structure, centrally dividing the respective rail into two adjacent tracks, each track being engageable by a wheel of a robot service device, characterised in that: each opening is surrounded along its width and length, by a raised lip provided by the rails (22a, 22b); wherein each rail in the first set of rails comprises a longitudinally extending dividing structure, centrally dividing the respective rail into two adjacent tracks, each track being engageable by a wheel of a robot service device, and wherein the dividing structure of each rail of the first set of rails comprises a series of raised dividers, each raised divider extending along, and parallel to, the full width of an adjacent opening, and wherein the dividing structure of each rail of the second set of rails comprises a series of raised dividers, each raised divider extending along, and parallel to, the full length of an adjacent opening; and wherein each raised divider of the dividing structure of each rail of the first set of rails extends no further than the width of the respective adjacent opening, and each raised divider of the dividing structure of each rail of the second set of rails extends no further than the length of the respective adjacent opening. each opening is surrounded along its width and length, by a raised lip provided by the rails (22a, 22b); wherein each rail in the first set of rails comprises a longitudinally extending dividing structure, centrally dividing the respective rail into two adjacent tracks, each track being engageable by a wheel of a robot service device, and wherein the dividing structure of each rail of the first set of rails comprises a series of raised dividers, each raised divider extending along, and parallel to, the full width of an adjacent opening, and wherein the dividing structure of each rail of the second set of rails comprises a series of raised dividers, each raised divider extending along, and parallel to, the full length of an adjacent opening; and wherein each raised divider of the dividing structure of each rail of the first set of rails extends no further than the width of the respective adjacent opening, and each raised divider of the dividing structure of each rail of the second set of rails extends no further than the length of the respective adjacent opening.