What is the difference between a low set and high set relay. Is it in relation to current setting or with time setting. What is its relation to an instantaneous relay.

You are correct. Low/high set is in relation to the current setting. The low-set instantaneous setting is typically made at a margin just above the maximum demand load current that a distribution or sub-transmission line would carry and is used as a sensitive fault detector or to trip on the first shot of reclosing to save a fuse from blowing for a temporary (weather/wild-life) fault on a distribution lateral or for close-on-to-fault protection of a transmission line. The high-set instantaneous setting is typically for protecting a transmission line and is set at a margin below the maximum remote terminal fault magnitude so that it does not over-reach and trip for faults outside the line protection zone. These are two of the common applications for low/high-set instantaneous functions, others might comment on additional protection applications.

Your comment is most welcome.

There are many examples of how to use Lo and Hi set elements
Consider overcurrent protection on the HV side of a transformer which has to grade with the OC protection on the LV side
Because of the TF impedance, the maximum, LV fault current is much less than the maximum HV fault current (per unit basis)
So we need to use a "Lo Set" inverse curve element (IDMT) from the rated HV current up to the max LV fault current as seen on the HV side in order to grade with the LV IDMT element.

But on the HV side, if we see fault current above the max LV fault current value, we know the fault is in the TF so we don't have to wait for the IDMT element to operate - hence the HV instantaneous Hi Set element is set at say 120% of the max LV fault current that would be seen on the HV side.

One final comment - setting of OC relays is a very specific skill taking all sorts of factors into consideration to make sure that it won't operate for 'normal conditions but also that it will operate for required conditions. This includes the specification of the CT class itself and the pick up settings, curves and time multipliers. I would suggest you get a proper grading study done my specialists with experience in the process.

The relay includes a high-set and low-set over current unit.
When the phase currents exceed the set start value of low-set stage I>, the overcurrent unit will start to deliver a start signal after a ~55 ms’start time. When the set operate time at definitetime characteristic or the calculated operate time at inverse definite minimum time (IDMT)characteristic elapses, the overcurrent unit will deliver a trip signal.
When the phase currents exceed the set start value of high-set stage I>>, the overcurrent
unit will start to deliver a start signal after a ~30 ms’ start time. When the set operate time
elapses, the overcurrent unit will deliver a trip signal.

Perhaps even simpler to understand ...
Lo Set is probably going to be set at say 105% - 120% of full load current - it may be instantaneous, Definite Time or IDMT depending on the grading requirements.

It is then possible to have additional stages - in the old days this was an instantaneous element only and was called the "HiSet" as it was set with say 500% - 1500% of full load current depending on the grading requirements.

But these days with numerical relays we can have several additional stages and they often can be also selectable as instantaneous, Definite Time or IDMT.

So whilst the concept of HiSet still exists, it is really just an additional stage or.element. I guess it would still be reasonable to refer to the highest setting element as the "Instantaneous HiSet"