Ostraka is a general term used to describe flakes of stone or pieces of pottery used as writing or drawing material. It is also extended to cover potsherds used for the same purpose. The Egyptians used ostraka for all sorts of purposes: keeping work records, making sketches of scenes and plans, practising writing, or simply for doodling. At sites such as Deir el-Medina and the Valley of the Kings they have been found in their thousands.
The examples from TT99 are somewhat more modest.
The most intriguing of these ostraka is the first one shown below (both sides), found in one of the shafts inside the tomb. Originally suggested to be a list of names, that explanation does not make sense when examined carefully. Our colleague Ben Haring in Leiden has come up with a proposal that this is actually the earliest example of a list of words in an alphabetic canon called Halaham which is known from the Bronze age onwards. For more detail, see his article, summarised here: http://www.nwo.nl/en/news-and-events/news/2015/gw/the-earliest-known-abecedary.html
Thomas Schneider in Canada has come up with a further approach to this, which is summarised in various articles in 2018: