Unicode 6.1 added a new Script_Extensions property, defined for a number of code points in the data file http://unicode.org/Public/6.1.0/ucd/ScriptExtensions.txt
# The Script_Extensions property indicates which characters are commonly used
# with a limited number of scripts, but with more than one.
# The property is provisional: values are expected to change over time as more information becomes available.
# For each code point, there is one or more property values. Each such value is a Script property value.
# For more information, see:
# UAX #24: http://www.unicode.org/reports/tr24/ and
# UAX #44: http://www.unicode.org/reports/tr44/
We want to use this property for more accurately identifying mixed-script hostnames in bug 722299 (or a follow-up to that bug). I don't know if font code would also want to use it.
*** Bug 844106 has been marked as a duplicate of this bug. ***