The "ContextStrings" tab is the sixth tab in "Project Settings".
A ContextString is a unique identifier for each page. It can consist of only letters and numbers and some limited characters such as a hyphen and underscore. FastHelp automatically creates the ContextString each time you add a new page. In doing this, it automatically removes spaces and gives an uppercase character to the first character of each word. If you do not like how the Context Strings are automatically created then you can control how they are created here.
In the "Replace with Spaces" section you can choose between removing the space or replacing it with a "-" or "_". You can also convert the whole ContextString to Upper or Lower case. And the final option is to add your own prefix. This is useful if you are integrating your help file into an application where there are certain syntax rules. For example, some C++ programmers use "IDS_" to prefix strings.