Project Properties are project specific items of information that can be re-used by reference throughout the project. An example of a Document! X Project Property would be the Copyright Notice property.
In addition to the standard properties defined for the Template you are using, you can define custom project properties that can referenced from anywhere in Topic content using a %% prefix and suffix. e.g. If you defined a custom project property called ProductName, in which you defined the current Help Systems product name, you could include the property in Topic content as follows:
Welcome to the %%ProductName%% Help System.
When you build, this would be filled out with the value you provide for the ProductName property in the Project Settings, or the Build Profile, e.g.
Welcome to the NightSky Professional Help System.