You are asked to supply your table split file, in the install guide, in the format "<TABLE>%<# SPLITS>".
However, this does not supply the table column to split by.
When splitting large tables, during the Table Splitting preparation phase (before you even start exports), R3ta can run for quite a while whilst is performs scans of the available INDEXES and then COUNTs the number of rows in the table(s) to be split.
It's trying to define the most apt column(s) to use for generating the WHR files which contain the query predicates.
I tried adding a specific column during the initial table splitter screens, where you can specify the column to use. However, this seems to be completely ignored.