There are two slits and two slots on EIS, are they just four types of grating?
You can go back and reference the paper published on this telescope. Specifically it is Culhane et al. 2007. This paper talks about the overall telescope, however the paper describing the optics is Korendyke et al. (2006). Unfortunately this optics paper is behind a paywall.
Within the Culhane paper, in section 3, the describe the slit mechanism as
Exposure times are controlled by a rotating shutter while a slit exchange mechanism can allow selection of four possible apertures — two spectral slits and two spectral imaging slots.
That's about as much detail as they give, except to note in table 1 that the slit and slot widths are 1, 2, 40, and 266 arcsec.
In the Korendyke optics paper, they give a bit more detail.
Light passing through the slit is dispersed and stigmatically reimaged by the toroidal grating onto two 1024 x 2048 pixel CCD detectors. The focus of the grating can be remotely adjusted with a mechanism. In flight, the mirror will be rotated in ~0.125 arcsec increments about the Y axis [solar north–south (N-S)] to sample different structures of the solar surface with the slit. High-resolution spectroheliograms (raster images) are formed by steadily moving the solar image in fine increments on the spectrograph slit and taking re-peated exposures. An interchange mechanism allows selection among two slits (1 and 2 arc sec wide) and two slots (40 or 266 arc sec wide). The slot observations of the solar disk produce spectrally pure images of large areas of the disk in bright solar emission lines with each exposure. The slot images exhibit a modest spatial blur along the dispersion direction.
For completion, they also include the diagram below of the optics.
From reading all of this, it appears to me that the various slit/slot widths are designed for taking different types of images/spectra, either of very focused, single-instance areas (such as with the slits) or else of large regions (using the slots).