If you set the accuracy on the parcel each line will inherit that accuracy level if left blank. So in other words, you do not have to set the accuracy of the lines if you want them to be the same as the parcel's accuracy. If you want to change the accuracy of a given line to make it higher or lower than that of the parcel, you will have to manually set that. Also, if you are digitizing the line, the accuracy will get automatically set to 6.
On a similar note, if you do not set the accuracy of the parcel the parcel will inherit the accuracy from the plan, as will the lines. This is useful when doing a construction since the Properties tab isn't available. You can leave the lines accuracy blank if you want each line to inherit the same accuracy as the plan.
