From Microsoft web site:

In a generic type or method definition, a type parameter is a placeholder for a specific type that a client specifies when they create an instance of the generic type. A generic class, such as GenericList listed in Introduction to Generics, cannot be used as-is because it is not really a type; it is more like a blueprint for a type.

In my case I wanted to have two different generic list names, generated from different classes, but those classes would be inherited from one class, something like this:

  class IDName
  {
    public Guid ID;
    public string Name;
  }

  class FirstIDName: IDName { }
  class SecondIDName : IDName { }
Then I need one method to fill those classes:
public static void AddToIDName<T>(List<T> iDNameList, string name) where T : IDName, new()
{
  iDNameList.Add(new T
  {
	ID = Guid.NewGuid(),
	Name = name
  });
}

Here notice constraint "where T : IDName, new()". I needed constraint new because of line: "iDNameList.Add(new T"

Whole example:

using System;
using System.Collections.Generic;

namespace GenericList
{
  class Program
  {
    static void Main(string[] args)
    {
      List<FirstIDName> firstIDNames = new List<FirstIDName>();
      List<SecondIDName> secondIDNames = new List<SecondIDName>();

      AddToIDName(firstIDNames, "firstIDNameTestOne");
      AddToIDName(firstIDNames, "firstIDNameTestTwo");

      AddToIDName(secondIDNames, "secondIDNameTestOne");
      AddToIDName(secondIDNames, "secondIDNameTestTwo");
      AddToIDName(secondIDNames, "secondIDNameTestThree");

      Console.WriteLine("FirstIDName");
      foreach (FirstIDName firstIDName in firstIDNames)
      {
        Console.WriteLine($"id: {firstIDName.ID}, name: {firstIDName.Name}");
      }

      Console.WriteLine("SecondIDName");
      foreach (SecondIDName secondIDName in secondIDNames)
      {
        Console.WriteLine($"id: {secondIDName.ID}, name: {secondIDName.Name}");
      }

      Console.WriteLine("");
      Console.WriteLine("The end");
      Console.ReadKey();
    }

    public static void AddToIDName<T>(List<T> iDNameList, string name) where T : IDName, new()
    {
      iDNameList.Add(new T
      {
        ID = Guid.NewGuid(),
        Name = name
      });
    }
  }
}