SVFR is a clearance which ATC may offer you, in a Control Zone, when you are not able to comply with the VFR requirements.
The way in which it is used varies from country to country. For example:
In the US, it is often used when the weather is below the VFR requirements for controlled airspace, but above the VFR requirements for uncontrolled airspace. An SVFR clearance can be requested, and may be offered, to get in/out of an airfield in controlled airspace.
In the UK, the most common time SVFR is used is in a Class A control zone - Heathrow, or the Channel Islands - where VFR flight is not possible, so the only way you can enter that airspace without having an IR is with a SVFR clearance. SVFR is also used to get in/out of Class D airports at night, because VFR is not allowed at night in the UK, and IFR is not allowed in controlled airpsace if you don't hold an IR/IMC rating.
Hope that clears things up.
FFF
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