This is how ffmpeg describes it:
‘-ss position (input/output)’ When used as an input option (before -i), seeks in this input file to position. When used as an output option (before an output filename), decodes but discards input until the timestamps reach position. This is slower, but more accurate. position may be either in seconds or in hh:mm:ss[.xxx] form. ‘-itsoffset offset (input)’ Set the input time offset in seconds. [-]hh:mm:ss[.xxx] syntax is also supported. The offset is added to the timestamps of the input files. Specifying a positive offset means that the corresponding streams are delayed by offset seconds.
So, what is the difference between the two when both are used as input options? Are they same when
-ss is used as