HC,
RCS means that
a) you are inside controlled airspace
b) you are receiving a surveillance service
Flying VFR inside CAS you will be provided with traffic information against other VFR, SVFR and IFR flights. No "separation" is established.
However to facilitate safe and orderly integration of VFR/IFR traffic inside class D and around airports, some forms of positive controlling are applied, even though they are not aimed at establishing "separation" per se, such as orbits, visual reporting, holding points, VFR/SVFR routes, flying not above/not below a certain level etc etc
I now quote CAP493 (Manual of Air Traffic Services) (some of it doesn't really apply to most ATC units with processed radar data, but anyway..)
Provision of Surveillance Services
1.1.1 Surveillance services comprise:
a) separation of arriving, departing and en route traffic;
b) vectoring;
c) position information to assist in the navigation of aircraft;
d) monitoring traffic to provide information to the procedural controller;
e) assistance to aircraft crossing controlled airspace.
1.1.2 Before a controller provides any of the above services he shall either:
a) identify the aircraft, using a method appropriate to the surveillance system in use;
or
b) have had the identity of the aircraft transferred from another controller.
The act of identifying an aircraft does not imply that a service is being given.
1.1.3 Surveillance systems may also be used to provide the following, whether or not the
aircraft has been identified:
a) Information on the position of aircraft likely to constitute a hazard;
b) Avoiding action;
c) Information about observed weather for pilots and other controllers; and
d) Assistance to aircraft in emergency
1.1.4 Surveillance services shall be provided to the maximum extent practicable to cover
the operational requirement subject only to workload, communications or equipment
capability.
1.1.5 Regardless of the type of airspace, or the air traffic service being provided, nothing
shall prevent a controller from taking action he considers appropriate if he believes a
risk of collision exists
1.2 Type of Surveillance Service
1.2.1 The airspace within which the aircraft is flying determines the type of surveillance
service available, as shown in the table below.
Type of Airspace - Surveillance Service
Controlled Airspace - Radar Control Service
Outside Controlled - Airspace Deconfliction Service or Traffic Service
1.2.2 Pilots must be advised if a service commences, terminates or changes when:
a) they are operating outside controlled airspace; or
b) they cross the boundary of controlled airspace
1.3 Radar Control Service
1.3.1 A Radar Control Service may be provided to aircraft operating IFR, Special VFR or VFR.
When providing the service controllers issue instructions to which:
a) pilots of aircraft operating IFR are required to comply; and
b) pilots of aircraft operating Special VFR or VFR will comply unless they advise the
controller otherwise.
NOTE: The manner in which VFR flights under Radar Control Service may be safely
integrated with the IFR traffic flow in the vicinity of aerodromes is described
in Section 3.
1.3.2 Before an aircraft enters controlled airspace the controller must establish which flight
rules the pilot will be operating under.
1.3.3 Before a Radar Control Service to IFR flights is terminated procedural separation must
be applied, except at ACCs when an aircraft will be entering an adjacent sector and:
a) a radar handover has been given; or
b) the conditions of any standing agreement have been met.
Hope this helps, although I understand it is not a very straight forward read.
Simple practical example:
IFR flight on short final ILS. VFR aircraft wants to transit through the overhead at the same time, at 1000' AGL.
By the book, the requirement would be to say to the VFR flight "GABCD, traffic is B747 2 mile final runway 27" and to the B747 "Traffic is cessna 152 east of the field etc etc routing east to west via the overhead at 1000' has you in sight (or not).
However this is such a critical phase of the flight that to avoid the unnecessary RT and to protect the landing traffic from a potential go around it is much easier and safer to hold the VFR transit one side of the field and as soon as there's a suitable gap in the sequence of the landing IFR traffic bring the VFR across.
This judgement call is made by the Tower man but if there's a busy sequence of inbounds coming in and no "gaps" to allow this transit the controller may suggest a rerouting, or ring the radar man and ask for a gap, he will then extend someone else downwind for a couple of miles etc etc so that there is enough room to bring the transit across.
Very simple and basic example but also shows how sometimes the reason for an instruction you are given by a controller may not necessarily be on the same frequency or possibly quite a few more miles down the line.
I realise it's a bit of a long convoluted message but it can be printed out and make for good toilet reading!!
regards