Difference between str.isdigit() and str.isdecimal() in Python 3

[Marco]

Hi all, because

"There should be one-- and preferably only one --obvious way to do it",

there should be a difference between the two methods in the subject, but 
I can't find it:

 >>> '123'.isdecimal(), '123'.isdigit()
(True, True)
 >>> print('\u0660123')
٠123
 >>> '\u0660123'.isdigit(), '\u0660123'.isdecimal()
(True, True)
 >>> print('\u216B')
Ⅻ
 >>> '\u216B'.isdecimal(), '\u216B'.isdigit()
(False, False)

Can anyone give me some help?
Regards, Marco