magento2 – What is the use of the "mapping" node in di.xml in Magento 2?

When you check Magento Payment Template Controls SpecificationFactory of construt

public function __construct ( Magento  Payment  Model  Checks  CompositeFactory $ compositeFactory, array $ mapping)
{
$ this-> compositeFactory = $ compositeFactory;
$ this-> mapping = $ mapping;
}

Second argument $ mapping array in the table you will get all your argument that you have defined in di.xml

EDIT

of construction $ this-> mapping you will find all the objects in the table that you have defined in di.xml.

For example, if you define then you have to define construction as

public function __construct ( Magento Payment Template Controls CompositeFactory $ compositeFactory, array $ mappingytest)
{
$ this-> compositeFactory = $ compositeFactory;
$ this-> mapping = $ mapping;
}

Right here $ mappingytest