The other documents are for a general understanding of this project. This document will cover some details, starting with the component on the PCB. For example, the PCB (printed circuit board) layout shows islands for each component lead to be soldered. These are also called "pads". Each pad is based on multiples of .100" spacing (1/100's of an inch). Another way to say is "number of units" a component will take up on the board. This is the normal "artwork" PCB layout for "old school" industry standards. Of course, newer PCBs are different, especially when using SMDs, for example mother board for PCs. The following pictures are examples and not necessarily this version of board but the same idea applies when explaining component placements.
The left image shows a resistor using 2 units, being the minimum it can be. Next to it is the board's layout equivalent. The middle one shows using 3 units. In this case allows one run to go between the component. The right image shows using 4 units. In this case allows two runs to go in between the component. Next to it is the board's layout equivalent for both 3 and 4 units. Another feature of 4 units allows the resistor to lay flat on the board. This prevents bending of the leads, due to mishandling of the board. More units can be used and is handy in the event several runs need to go under the resistor.