I believe the DFDL spec does not specify whether the prefixLengthType for an element of lengthKind 'prefixed' can have assertions, and if those assertions are evaluated, and when. Consider this example below. We really want the assert on the bufferPayload type to be evaluated before the length computed from the prefix is used as a length because we'd like good diagnostic behavior that points out that the prefix value is way out of realistic range. In many formats we see 4-byte length prefixes, but the range of values for specific fields is relatively tiny, often even just 1 to 100. Enforcing this via the assert expressed on the prefixLengthType's simpleType definition makes this easy. I could go either way on whether a prefixLengthType can carry a discriminator. But an assert should definitely be allowed. ... <complexType> <sequence> <element name="payload" type="xs:hexBinary" dfdl:lengthKind="prefixed" dfdl:prefixIncludesPrefixLength="no" dfdl:prefixLengthType="p8:bufferPayload"/> </sequence> </complexType> ... <simpleType name="bufferPayload"> <annotation> <appinfo source="http://www.ogf.org/dfdl/"> <dfdl:assert>{ dfdl:checkConstraints(.) }</dfdl:assert> </appinfo> </annotation> <restriction base="p8:unsignedLong"> <minInclusive value="0"/> <maxInclusive value="32768"/> </restriction> </simpleType>