I just found the answer from
COMMISSION DECISION 2009/767/EC
The meaning of
http://uri.etsi.org/TrstSvc/eSigDir-1999-93-EC-TrustedList/SvcInfoExt/RootCA-QC additionalServiceInformation extension is stated directly in that document.
As a general default principle, for a listed CSP in the Trusted List
there must be one service entry per single X.509v3 certificate for a
CA/QC type certification service, i.e. a Certification Authority
(directly) issuing QCs. In some carefully envisaged circumstances and
carefully managed conditions, a Member State Supervisory
Body/Accreditation Body may decide to use the X.509v3 certificate of a
Root or Upper level CA (i.e. a Certification Authority not directly
issuing end-entity QCs but certifying a hierarchy of CAs down to CAs
issuing QCs to end-entities) as the Sdi of a single entry in the list
of services from a listed CSP. The consequences (advantages and
disadvantages) of using such X.509v3 Root CA or Upper CA as Sdi values
of TL services entries must be carefully considered and endorsed by
Member States. Moreover, when using this authorized exception to the
default principle, Member State must provide the necessary
documentation to facilitate certification path building and
So the meaning and purpose of that extension is to clearly indicate (As a warning?) that the way the X.509v3 certificate is being associated to the TSP service is not under the default principle of including the direct CA for end user accredited Qualified certificates.
This would mean that in the original example without including the
http://uri.etsi.org/TrstSvc/eSigDir-1999-93-EC-TrustedList/SvcInfoExt/RootCA-QC additionalServiceInformation in the TSP, constructing a certification path including B and C certificates wouldn't be appropiately supported by that TSL.