ZF - Zero filling (nmrPipe function)
From NMR Wiki
Zero filling function adds zeros to the data vectors to either increase digital resolution or prepare the data for the Hilbert transformation.
The function can also be used to truncate data vectors.
Zeroes are added or points removed on both real and complex part (if present) of data vectors.
Note that final data size of n = 2i, where i is integer speeds up the fourier transformation.