The Parts List (or PLI) is a box that shows what parts are added to your model in a given step. LEGO originally did this in its Technic building instructions and nowadays it is used more and more in the other product lines as well. The dialog has two tabs.
Uncheck Show Parts List to not show a parts list per step.
Using a Background images works the same as the Page background. Using an image can look nice. This is what LEGO did in the past for its Technic instructions (very light blue grid).
For a border you can choose 3 styles. Square Corners is default, but you can also use round corners or no border. Experiment with these.
The margins around the box that contains the PLI.
With the Constrain settings you can determine the default shape of a PLI. You can change the global settings here, but it is not recommended. LPub finds out what the best possible setting is and fits the parts automatically. You can also format individual PLI's per step. This will be explained later.
In the Contents tab you can setup the scale, fonts and colors used in the PLI.
It is recommended to set the value of the scale of parts to 75% of the scale of the Assemblies. For example if your Assemblies are at 100% use the value 0,750000 (that is 75%). Leave the Lattitude and Longitude for what they are and the Margins are good at 0.0500.
If you use Submodels you should leave the box Show in Parts List unchecked.
Use a small font for Part Counts like size 18 or so and keep it black.
Use font size 16 for Part Length and make it blue. This length is the length in studs notated for example with Technic axles and beams.