The Link connection can only be used with ELT components. These connections transfer table schema information to the ELT mapper component in order to be used in specific DB query statements.
The Link connection therefore does not handle actual data but only the metadata regarding the table to be operated on.
When right-clicking the ELT component to be connected, select Link > New Output.
Warning: Be aware that the name you provide to the connection must reflect the actual table name.
In fact, the connection name will be used in the SQL statement generated through the ELT Mapper, therefore the same name should never be used twice.
For more information, see the documentation for Talend ELT components.