default is a reserved word that can be used in Frequency fields (as well as in some other fields, such as BankSize). Your second example (SignalProcessor halfSampleRate) is a message sent to the object SignalProcessor. This one can probably be superceded nowadays by using the signalToHz message.
Because of the way things have evolved over time, there's no central list of the "reserved words" and other handy messages (although for the Capytalk messages, you can always hover the cursor over the purple text to read what it means). You can also open the Capytalk Reference from the Help menu to explore and look at examples.
In Kyma X Revealed there's an appendix called Smalltalk Quick Reference on page 375 that includes things like that message to SignalProcessor.
Would you be willing to keep track of things like these in a list as you encounter them? That way, we could explain them as they come up in context with the end result being a document that we could share here. Are you open to this idea?