Effectively one built to very exacting tolerances.

For example, ive just built a bottom end for a 450bhp Vauxhall engine, an example of the blueprinting ive done is i have decided on a piston to bore clearance of 3.5 thou, and thats EXACTLY (to within a tenth of a thou) what the clearance is on every single bore all the way up and in every direction, likewise all the weights of the rods and pistons are as exactly the same as i can measure down to.

So basically, it just means that you built it to an exact specification rather than within a wide tolerance like on a normal cheap and cheerful rebuild.