What are the black and white boxes on survey plan?

These boxes indicate the presence of survey monuments - 2 or 4 foot long iron survey bars that surveyors drive into the ground to physically mark boundary points. 

The solid black squares indicate an existing iron bar that the surveyor found and is confirming the presence and location of.

The white squares with black outline indicate that the surveyor put a new iron bar in the ground to mark that boundary point.