When a synth is created from a time stamped osc-bundle, it starts calculation at the next possible block (normally 64 samples). Using an OffsetOut UGen, one can delay the audio so that it matches sample accurately.
For some synthesis methods, one needs subsample accuracy. SubsampleOffset provides the information where, within the current sample, the synth was scheduled. It can be used to offset envelopes or resample the audio output.