This is a configuration pragma that requests implicit packing for packed arrays for which a size clause is given but no explicit pragma Pack or specification of Component_Size is present. Consider this example:
type R is array (0 .. 7) of Boolean; for R'Size use 8;
In accordance with the recommendation in the RM (RM 13.3(53)), a Size clause does not change the layout of a composite object. So the Size clause in the above example is normally rejected, since the default layout of the array uses 8-bit components, and thus the array requires a minimum of 64 bits.
If this declaration is compiled in a region of code covered by an occurrence of the configuration pragma Implicit_Packing, then the Size clause in this and similar examples will cause implicit packing and thus be accepted. For this implicit packing to occur, the type in question must be an array of small components whose size is known at compile time, and the Size clause must specify the exact size that corresponds to the length of the array multiplied by the size in bits of the component type.