I was reading https://github.com/JuliaComputing/PooledArrays.jl which describes a PooledArray. I tried to follow the source code and looked at the constructor, I gather that a pool is the same as a level in R/Stats speaking which is the same as a category in SAS/stats speak.

In uni stats courses it’s referred to as factor levels, which are basically the unique possibles values in a categorical variable. Also in a pooledarray representation there is a ref which is of integer type *RA*, it acts as pointers to the list of possible values very much like how R’s factors work.