This is an upgraded version of the original IS-11 computer. Basically, it has a much wider LCD screen (80 x 25), which means it's 6 times larger than the original IS-11 screen.
The IS-11c is more wordprocessor and communication oriented than the IS-11, that's why wordprocessing and communication software are included, as welle as a Modem.
The DB-PIPS package (spreadhseet, graphics, file manager) delivered with the IS-11 is there available on a ROM cartridge.
The RAM is also bigger : 80kb instead of 32kb for the original IS-11.
The microcassette recorder still acts as virtual disk-drive (indexed access) and one tape can old as much as 128kb.