In the Luxeon LED datasheet under Solder Pad Design section there is a diagram with 3 crosses around the edges of the LED. They are identified as "4x Soldermask Opening (hatched area)" but I am not entirely sure what it is exactly or why it is used. What are they, what do they achieve and how should they be drawn in Eagle or other software?
Soldermask openings are CAD layers usually meant to specify to the PCB fabrication house that they shouldn't put any other covering on the bare copper so that when the soldermask layer is applied, the solderpaste can be placed on the direct copper.
In the case of the cross mark you mention however, the opening is probably specified to provide an alignment mark for the pick-and-place machine that will use the cross to figure out when the part is aligned properly before placing it down. The machine probably need the bare copper to get a good image of the alignment mark
Soldermask is used to cover the copper to prevent solder from adhering to it (hence its name, "soldermask"). Soldermask is the green stuff you see on most PCBs, though it can be just about any color (red, purple, blue, white, black, etc). By creating a soldermask cutout you will allow copper to show through the mask. This is often used for fiducials, which are used as optical markers for the position of components and used by pick-and-place machines to properly align components. I expect that's what the crosses are used for in the component layout you posted. They are either fiducials or, more likely, indicators of polarity. Having a non-symmetrical mark to indicate where the cathode and anode are can be useful for the assembler as well as anyone debugging the board, reworking it, testing it, etc. I can't quite tell, but there is also the possibility that they are being used as actual solder pads. I see the LED has matching marks, but I can't tell if those are metal tabs or just marks on the case. I'm pretty sure the crosses in the PCB layout are just there to make sure the assembler aligns the LED correctly on the board.