Value,Description,Reference
custom,"The element content is free-form and of the data type
        specified by the dtype attribute.  If this value is selected,
        then the dtype attribute MUST be set.",[RFC7970]
cpe,"The element content describes a Common Platform
        Enumeration (CPE) entry per [[NIST.CPE]].",[RFC7970]
swid,"The element content describes a software identification
        (SWID) tag per [ISO19770].",[RFC7970]
ext-value,"A value used to indicate that this attribute is
        extended and the actual value is provided using the
        corresponding ext-* attribute.  See Section 5.1.1 of [RFC7970].",[RFC7970]