What are the system fields?

In the Prospect field, there are fields which are already present called "System fields" and those that are made up by the users as per specific needs are called "Custom Fields".

Later, you can use these fields while writing the cold emails in the form of "Merge Tags". There are two types of system fields that are "text" and "number". 

You can add Fallback Text to your field when field data is empty.