Last summer I asked a similar question of Keith in the tomato growing forum. Here's a bit of what he sent me plus a diagram he sent. I e-mailed you the whole thing. I thought he did a great job of explaining!

George
Tahlequah, OK
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I know of no hard and fast definition of "beefstake". Usually refers to more "meat" in the tomato as opposed to seed or locular cavity.

My understanding of it is that it tends to be a combination of shape and locule arrangement.

Locules are the areas that hold the seeds and gell. There are 5 in this example arranged in a wheel like manner around a center.

There are bilocular fruits such as on cherry types:

And their multilocular types were there are 3 or more locular cavities usually arranged around a center point like spokes on a wheel.

Beefstakes tend to have their multiple locular (multilocular) cavities in irregular patterns sometimes with no definable center as the pictures above. The "meat" or flesh area, called "pericarp: and "septa" in the line drawing above, tends to be thicker and generally occupies more space than the locules with beefstake types.