As a SVFR controller for 2 decades I couldn't really better the excellent summary in the Pooley's Flight Guide. Take a look at that. Remember that SVFR is a concession where VFR flight is not permitted (eg in Class A or Class D below VMC) and IFR flight is not possible for whatever reason. SVFR must not hinder IFR traffic and separation must be provided. So it is not IFR, nor VFR, it is a SPECIAL visual flight clearance under Special Rules. Remember that SVFR flight must remain clear of cloud and in sight of the surface. SVFR flights are absolved from the '1000 ft Rule' of Rule 5 (the low flying rule) to facilitate SVFR compliance at low levels but you must still be able to 'alight clear' etc. Non IMC capable pilots require 10km minimum flight visibility. This is reduced to only 3 km for IMC capable pilots. Because of the requirement to remain clear of cloud and in sight of the surface it is often impractical for ATC to assign a vector therefore pilot nav to ensure the lateral separation must be accurate. Where and when VFR is permitted in a CTR (eg. Class D) you may still request Special VFR to secure the bonus of separation should you require it!