From the IG:

SHALL contain exactly one [1..1] componentOf (CONF:1198-7595).

• This componentOf SHALL contain exactly one [1..1] encompassingEncounter (CONF:1198-7596).
• This encompassingEncounter SHALL contain at least one [1..*] id (CONF:1198-7597).

• This encompassingEncounter SHALL contain exactly one [1..1] US Realm Date and Time (DT.US.FIELDED) (identifier: 2.16.840.1.113883.10.20.22.5.3) (CONF:1198-7598).
• This effectiveTime SHALL contain exactly one [1..1] low (CONF:1198-7599).

The (custom-written) schematron rule:

<sch:assert id="a-1198-7598-c" test="(cda:componentOf/cda:encompassingEncounter[count(cda:effectiveTime)=1]) and (string-length(cda:componentOf/cda:encompassingEncounter/cda:effectiveTime//@value)&gt;=8)">This encompassingEncounter SHALL contain exactly one [1..1] US Realm Date and Time (DT.US.FIELDED) (identifier: urn:oid:2.16.840.1.113883.10.20.22.5.3) (CONF:1198-7598).</sch:assert>

This rule is asserting that the encompassingEncounter MUST have a @value, which goes beyond the conformance statement. Sending a nullFlavor at this location would seem to meet the conformance criteria, but the schematron treats this as an error.