Difference between Bidi_Class 'R' and 'AL'

UAX #44, Table 13 ("Bidi_Class Values") includes the following
descriptions:

R - Right_To_Left - any strong right-to-left (non-Arabic-type) character
AL - Arabic_Letter - any strong right-to-left (Arabic-type) character

But I can't find any definition, here or elsewhere, of what constitutes
an "Arabic-type" or a "non-Arabic-type" letter.

Looking in UnicodeData.txt, I see that Arabic, Syriac, and Thaana
letters are assigned a value of 'AL', while other RTL letters, including
Hebrew, N'Ko, Samaritan, Mandaic, and some archaic scripts in plane 1
are 'R'. Clearly, shaping behavior and ligation isn't what makes a
letter or script "Arabic-type" or "non-Arabic-type."

How would I make this distinction for an arbitrary letter or script,
other than by association with an existing letter or script already
designated as one or the other? (Yes, I admit, I am thinking about RTL
scripts in CSUR.)

--
Doug Ewell | Thornton, Colorado, USA | RFC 5645, 4645, UTN #14
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