Blueprint

The Blueprint modifier produces blueprint-like strokes using either circular, elliptical, or square contours. A blueprint here refers to those lines drawn at the beginning of free-hand drawing to capture the silhouette of objects with a simple shape such as circles, ellipses and squares.

Shape

Which base shapes to use for this blueprint: Circles, Ellipses or Squares.

Rounds

How many rounds are generated, as if the pen draws the same stroke several times (i.e. how many times the process is repeated).

Random Radius, Center

For the Circles and Ellipses shapes. Adds some randomness to each round in the relevant aspect. Using more than one round with no randomness would be meaningless, as they would draw over each other exactly.

Backbone Length, Backbone

For the Squares shapes. The first adds some extra length to each edge of the generated squares (also affected by the second parameter). The second adds some randomness to the squares.

Note that the Min 2D Length feature from the Strokes settings is quite handy here, to avoid the noise generated by small strokes...