First of all the FDA starts with describing how the tuna meat looks like after it is pre-cooked and cleaned :  The optional forms of processed tuna consist of loins and other striated muscular tissue of the fish. The loin is the longitudinal quarter of the great lateral muscle
 
Then they continue to describe what the different styles or forms of packing canned tuna are :
Solid or solid pack consists of loins freed from any surface tissue discolored by diffused hemolyzed blood, cut in transverse segments to which no free fragments are added. In containers of 1 pound or less of net contents, such segments are cut in lengths suitable for packing in one layer. In containers of more than 1 pound net contents, such segments may be cut in lengths suitable for packing in one or more layers of equal thickness. Segments are placed in the can with the planes of their transverse cut ends parallel to the ends of the can. A piece of a segment may be added if necessary to fill a container. The proportion of free flakes broken from loins in the canning operation shall not exceed 18 percent.
   
Chunk, chunks, chunk style consists of a mixture of pieces of tuna in which the original muscle structure is retained. The pieces may vary in size, but not less than 50 percent of the weight of the pressed contents of a container is retained on a \1/2\-inch-mesh screen.

Flake or flakes consist of a mixture of pieces of tuna in which more than 50 percent of the weight of the pressed contents of the container will pass through a \1/2\-inch-mesh screen, but in which the muscular structure of the flesh is retained.

Grated consists of a mixture of particles of tuna that have been reduced to uniform size, that will pass through a \1/2\-inch-mesh screen, and in which the particles are discrete and do not comprise a paste.

Information taken from : Code of Federal Regulations]
[Title 21, Volume 2]
[Revised as of April 1, 2002]

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