The post plan illustrates the building footprint including the location of door posts, interior posts, and corner posts. Note the measurement of each post spacing is different. Corner posts are measured from the outside of post, interior posts are measured to the center, and door posts are measured to the inside of the post
The post plan will be 3" shy of the overall building measurement to provide room to nail the skirt board, girts, and truss supports to the outside of the posts. These members will add 1 1/2" on all sides and make up for the inset corner post spacings. So, a plan that has 9' post spacings will have 8' 10 1/2" corner bay measurements and 9' measurements for interior bays of the plan.
The squaring diagonal measurement on the bottom right of the drawing is useful during the staking and string line setup phase required prior to digging your holes. See squaring-building-lines.html for more information regarding the squaring diagonal.