SFTD :SFN and Frame Timing Difference
In order to understand SFTD measurement function you need to understand 5G NSA Architecture where a UE is connected to e-NB and g-NB simultaneously to Provide 5G services.
SFTD measurements are supported only in the EN-DC scenario for 5G NSA Network architecture, in which the UE has dual connectivity with a 5G NR network and an LTE network simultaneously.
In case of SFTD measurements, the UE is required to measure the timing difference between the Primary Cell (PCell) of the LTE network and specifically the Primary Secondary Cell (PSCell) or you can say g-NB of the 5G NR network.
SFTD is configured for NR cell(s) via Radio Resource Control (RRC) signalling When it is not possible to set SMTC window for NSA UE, NR defines SFTD measurement Function where the UE Measures the Timing Difference of the SFN and the frame boundary between the serving cell and candidate cell and report it to the serving gNB ,The Base Station configures SFTD measurements for NR UE and its applicable for RRC_CONNECTED intra-frequency measurements only.