What is a SRF? (This is used in AC power distribution system with isolation transformer)
SRF = Self Resonant Frequency (as Dan D may or may not have suggested :-) ).

SRF is NOT a component or physical item, even though it appears to be one from your diagram. The SRF is the frequency where the capacitive and inductive elements in the system are of equal magnitude and opposite sign. This leads either to very low or very high impedances depending on configuration and point of measurement. The effect is that the system does not work as intended. Keeping SRF well away from the operating frequency is a key design requirement.

SRF is probably shown on the diagram as a reminder of its importance in the context being discussed.



From here - Transformer testing - page 14


Practical inductive components are not perfect inductors; they have stray 
resistances and capacitances associated with them. For certain components, 
especially those with a low inductance value, the impedance of the stray 
capacitance can become significant when compared to that of the 
inductance.   

XL = 2πfL XC = ½ πfC   

The frequency at which the inductive impedance equals the capacitive 
impedance (XL = XC) is known as the self-resonant frequency (SRF) of the 
component. At this point, the phase angle of the impedance (which can be 
measured using the ANGL test) is zero. 

At test conditions where the 
frequency is low enough for problems with capacitive impedance to be 
negligible, the phase angle will be positive and close to 90 degrees. 




Brief but useful comment - end of 1st page here



Several useful transformer related formulae  - several references to SRF 
This is a page on the Tesla Coil Information Archive site and is relevant but biased towards Tesla coil related applications.