thickness of diffusion layer

in electrochemistry
https://doi.org/10.1351/goldbook.T06342
The distance from the electrode where the ratio $\frac{c\,-\,c_{\text{e}}}{c_{0}\,-\,c_{\text{e}}}$ reaches a given value. (ce is the interfacial concentration, c is the concentration, c0 is the value in the bulk solution.) If this ratio is selected as, say, 0.99 the corresponding thickness of the diffusion layer is denoted by δ 0.99.