Transformer Restricted Earth Fault - REF

On the delta side you use the term BEF but I would still simply call it REF - the "B" doesn't necessarily indicate a differential type protection as there are other instances where unrestricted earth fault protection is used as back up in the hope that something else would operate first. If there is an ef on the TF delta, the REF would be considered as the principle, not back up, protection (in the absence of TF diff 87T)

But to the issue of what is SEF.
The S is most definitely for Sensitive. So what does that mean.
First we are talking about unrestricted schemes

EF detection can be provided by 3 means:

First using a residual connected relay in a 3 phase CT system - if your imagination can stretch this into a drawing with star connections at each end:
|-AphaseCT---------AphaseOCRelay-|
|-BphaseCT---------BphaseOCRelay-|
|-CphaceCT---------CphaseOCRelay-|
|------------------------------------EFRelay-|

This is not generally considered to be "sensitive" since firstly the EF setting must be above the max single phase current without a fault on the system - as a worst case that could be rated line current for single phase load but practically would be 10-20% of rated current for reasonably balanced loadings.
However more significantly is that the EFFECTIVE setting for this configuration - the value of primary current that needs to flow to cause operation is much higher than the relay setting.
Suppose teh CTs have an excitation/magnetising current Im of 5% and teh EF relay is set Ir at say 10%
The effective setting as secondary amps is given by
Ieff = Ir + nIm
n being the number of CTs, 3 in the case, 4 if a 4 wire system
So you can see that although set at 10% the effective setting is actually 25% minimum current to cause relay operation.

The second way with more sensitivity is to use a single CT on the neutral point to earth connection. This can be "sensitive" since the CT ratio and the relay setting can be set much lower ignoring rated line current, with the combination giving say 5 - 10% of rated current detection just to allow for normal transients and unbalances, but more importantly there is only 1 excitation current so a setting of 5% means the effective setting is still only 10%

the third method which is probably the most sensitive is to use a core balanced CT but this is really only practical on cables. All phases are passed through the one CT (and also if there is a 4th wire neutral), taking particular care if there is an earthed cable sheath which must be looped back through the core balanced CT. The effect of this is to eliminate all 'natural' unbalance currents that appear as earth currents and so extremely sensitive settings can be achieved and the CT can be made with very low Im.