Parse Structured rule can be used in conjunction with a Service File rule or the Apply-Parse-Structured method to import structured data to the clipboard. For example, you can import fixed-format flat files from various external sourced and platforms. For more information go through the Pega Help over here.
Whereas Activities automate processing. Activities contain a sequence of structured steps. Each step calls a method rule or contains a control instruction such as Call or Branch. Each activity has a type that characterizes its purpose and affects which other rules can call it. For more information go through documentation over here.
So i don't think both are interlinked concepts as per the above explanation. Hope that answers your query.