[...] we are chosen to indicate formally the fact that a feature value is redundant in the systematic phonemic matrix of a morpheme by replacing that feature value by "0" in the dictionary matrix of the morpheme and by including an MS rule which changes that "0" to the proper feature value.
- Stanley (2004), Pag. 61

Both the symbol "0" and the absence of a symbol, i.e. a blank, are used in the literature to indicate redundant feature values.
- Stanley (2004), Pag. 89