light - What does the filter name I+z' mean

They must be referring to two different filters:

• the (Bessel) I filter which has a central wavelength of $lambda simeq 800,mathrm{nm}$ and a width of $Deltalambdasimeq 150,mathrm{nm}$, and


• the (Sloan) z' filter, which has $lambda,Deltalambda simeq 970,255,mathrm{nm}$.

Hence their transmission is in the near infrared ("NIR"), and they're both above 700 Å.

The exact transmission curves vary a little depending on the producer, so if you want to calculate exact magnitudes, for instance, you need to find the transmission curves for the telescope in question. From the TRAPPIST equipment website, it seems they use filters from Astrodon. Astrodon don't seem to provide filter curves for these exact filter, but if you're okay with a good approximation, you can find the curves here:
Bessel I and
Sloan z'.