Latching current is another name for gate trigger current -- it's a measure of
the minimum current you have to apply to the gate of an SCR to be guaranteed it
will turn on (given that there's a minimum voltage between anode and cathode).
Holding current is the specified minimum current that must be flowing from
anode to cathode (with no gate current) and still be guaranteed that the SCR
will not turn off.
As an example, the latching (gate trigger) current for a 2N5062 (logic level
SCR in TO-92 pkg) is 350 uA over the entire temp range, and the holding current
is10 mA over temp.
latching is the required minimum current on the gate that should
be used to turn it on.
the holding current is the minimum current it can be reduced down
to before it opens the path between the cathod and anode.
usually there is also a minimum voltage rating due to diode
cut off voltages internally.
for example many scr's won't latch if the voltage across the
anode and cathod is below 1.0, there are the higher power types that
even have a much higher voltage drop out too.