What does the filter name I+z' mean On the TRAPPIST telescope, the filter wheels are described to have a filter with the following description:


  NIR Luminance I+z' (>700nm)


After a bit of googling, I wasn't able to find what the specifics of this filter are (its FWHM and mean wavelength).

How would I go about finding these qualities of this filter?
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 $\Delta\lambda\simeq 150\,\mathrm{nm}$, and
the (Sloan) z' filter, which has $\lambda,\Delta\lambda \simeq 970,255\,\mathrm{nm}$.


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'.