Custom Speech is for when  you use unique names or words on the call that you want the system to pick up accurately in the transcription. 

Topic allows you to create group of words you want to filter for later use.  

For example:
You can create a Topic called competitors and list out your competitors.  When the meeting transcript gets generated, you can then filter to the competitor category in the notepad and check the times when your competitors are mentioned.

Both features are available starting from the Pro Tier Plan.

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