The Degrees of the Royal Arch are seven in number according to the Laws and Constitutions of the Supreme Grand Royal Arch Chapter of Scotland.
Although from a joining point of view you need only concern yourself with the first three.
Since 1860 the Mark Degree has been under the jurisdiction of both the Grand Lodge of Scotland and the Supreme Grand Royal Arch Chapter of Scotland.
Craft Lodges holding their Charter or Warrant from the Grand Lodge of Scotland are permitted to recognise the Mark Degree as part of the 2nd. or Fellow Craft Degree on the condition that they only confer it on Master Masons. The Supreme Grand Chapter works this degree as the first in Royal Arch Masonry but recognises Mark Masters of Craft Lodges which work under the Grand Lodge of Scotland.
A Brother that has had the Mark Degree conferred upon him in a Craft Lodge is only required to affiliate to the Chapter.
The last four of the above degrees - those beginning with Installed - are conferred on Companions that are nominated and elected as Third, Second and then First Principal, who is also lucky enough to have the Mark Master conferred upon him as well.