Rankine, where kelvin is Celsius but it starts at absolute zero, is fahrenheit but starts at absolute zero.
The main thing that separates Kelvin and Rankine is that Rankine is more "precise" which is to say, using Celsius and Fahrenheit as examples
22 Celsius is 71 Fahrenheit. 23 Celsius is 73 Fahrenheit. Fahrenheit is more precise in that the space between 2 adjacent whole numbers in it is a smaller temperature change than than the space between 2 adjacent whole numbers in Celsius. The same is true of the difference between Rankine and Kelvin.
As far as I’m aware Ra is just another way of symbolising rankine. There are 2 other systems that start with R, one is romer which is Ro and raeumur which is Re. From what I know you use R for all three when each is alone or with C/F and use the second letter to tell the difference when comparing them
The main thing that separates Kelvin and Rankine is that Rankine is more "precise" which is to say, using Celsius and Fahrenheit as examples
22 Celsius is 71 Fahrenheit. 23 Celsius is 73 Fahrenheit. Fahrenheit is more precise in that the space between 2 adjacent whole numbers in it is a smaller temperature change than than the space between 2 adjacent whole numbers in Celsius. The same is true of the difference between Rankine and Kelvin.