Any code point (assigned or not) with combining class of zero (ccc=0).

• Note that ccc=0 is the default value for the Canonical_Combining_Class property, so that all reserved code points are Starters by definition. Noncharacters are also Starters by definition. All control characters, format characters, and private-use characters are also Starters.
• Private agreements cannot override the value of the Canonical_Combining_Class property for private-use characters.