Hi Etienne,
Yes, in short, both SPPT and SKEB are available in OpenIFS 43r3. You'll need a small patch for 43r3v1 as there's some code (inadvertently) missing to enable SKEB. There is also mention of the 'SPP' scheme in the code. This is not a typo, but a replacement for SPPT which, at 43r3, is in the early stages of development and should not be used. Operationally, at 43r3, SKEB was 'turned' right down and had only a small effect on the spread. The advice I've had from the stochastic experts here is that for OpenIFS 43r3 it's best to use only SPPT.
Documentation.
The scientific detail for the ensemble schemes is contained in the IFS CY43r3 manual which you can find here: https://www.ecmwf.int/en/elibrary/17737-part-v-ensemble-prediction-system
The user guide is lacking some practical examples of using ensembles with OpenIFS which I'm aware of and on my list to do as soon as possible. In the meantime, there is some helpful information from the OpenIFS workshops which have used ensembles. See OpenIFS 2017: Creating a new OpenIFS experiment#Runningtheforecastexperiment . Much of this is still valid for 43r3.
It's important to be bear in mind that the parameters for the SPPT scheme operationally are chosen to give a forecast spread that matches the error based on many simulations of the 10 day forecast. The seasonal prediction system also uses SPPT but a different set of SPPT parameters for that reason.
Hope that helps.
Best regards, Glenn