From QSPR Wiki
The System of units for a property that do not have any associated physical dimension, see for more details Dimensionless_quantity. Examples of properties that have this system are, e.g. 1-octanol/water partition coefficient, % of drugs absorbed.
It is important to understand that the data can still have different units, e.g. values can be as ratio, log of ratio, logit and so on. Thus, the user should pay particular attention which specific dimension of the Dimensionless system is assumed in the article, e.g. whether the reported value corresponds to ratio, % or log of ratio of some property.
The default value for this system is log10, i.e. the logarithm of 10 of a property.