L2/02-407
[Excerpt from TUS 4.0 draft]

Default Property Values

Implementations need specific property values for all code points,
including those code points that are unassigned. To meet this need, the
Unicode Standard assigns default properties to ranges of unassigned code
points.

D11 <i>Default property value</i>: For a given Unicode property, the
value of that property which is assigned, by default, to unassigned code
points or to code points not explicitly specified to have other values
of that property.

In some instances the default property value is an implied null value.
For example, there is no Unicode Character Name for unassigned code
points, which is equivalent to saying that the Unicode Character Name of
an unassigned code point is a null string.

In other instances, the documentation regarding a particular property is
very explicit, and provides a specific enumerated value for the default
property value. For example, "XX" is the explicit default property value
for the Line Break property.