How to add field groups in sharepoint list item?

Question

SharePoint

Plz tell me, I need to add a group of fields for a separate element of the Share point 2013 list
. I create a regular list called "Register of contracts". It will contain contracts and various attributes of these contracts (number, date of conclusion, amount, etc.).
One of these attributes is the specification of such specifications, a contract may have several such specifications, each with its own attributes (number, date of conclusion, amount, etc.). Therefore, having fallen from the list into a specific contract, it should be possible to add (with a plus, for example) a group of fields (specification) at once.
It turns out inside the list element, instead of one of the fields, is also a list.
How to do it?