Double ferrule or Single ferrule hydraulic compression fittings?

What is the Difference Between Double Ferrule and Single Ferrule

Double Ferrule Compression Fittings • Projectmaterials

N2403-10-10-SS Union - Compression Fittings, N2403 Series

Single Ferrule Compression Fittings Stainless Steel ISO8434-1/DIN

Single Ferrule Hydraulic Fittings, Size: 1/4 to 2 at Rs 50/piece

Difference Between Double & Single Ferrule Fittings

Twin Ferrule Compression Tube Fittings Suppliers Manufacturers

Instrumentation Tube Fittings, Instrumentation Ferrule Fittings

Single Ferrule Vs Double Ferrule Fittings, by Rohan Vaghela

Inconel 625 UNS N06625 Ferrule Fittings Manufacturer

Why do people prefer the Double ring fitting or Single Ring?