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A great and detailed explanation can be found here: LCLS 1 Event Generator for Dummies
In summary, the main LCLS-I timing is based on a 360 Hz frequency, which is divided in 6 time slots (TS), each running at 60 Hz, but with a different phase. The 120 Hz beam is a combination of slots TS1 and TS4, while the other ones are "empty".
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The sync marker defines on which phase the sequence can be started. For HXR, the sync marker follow the event codes 4X.
A sync marker of 120 Hz thus means that the sequence will start on the next 120 Hz tick (TS1 or TS4). With a sync marker of 60 Hz or below, the sequence is guaranteed to start on TS4. This is important to ensure a deterministic start of the sequence when multiplexing.
When running a sequence in a loop (N times or repeat forever), the sync marker determines the start for each loop. This means that it will limit the frequency at which the sequence will run. For example, if a sequences that could run at 30 Hz (4 delta beams total, see below) is run with a 10 Hz sync marker, it will only start on a 10 Hz frequency and thus run at 10 Hz.
Lower rate operation
When using the sequencer to operate at lower rep rate (60 Hz and below) or on a custom sequence of beam, one wants to make sure that the DAQ is not synchronized with the 10 Hz shot sent to SXR. That means DAQ must be triggered only on TS4.
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