If true, the schemas of the tables will be first unified with fields of the same name being merged, according to #GArrowTableConcatenateOptions:promote-nullability, then each table will be promoted to the unified schema before being concatenated.
Otherwise, all tables should have the same schema. Each column in the output table is the result of concatenating the corresponding columns in all input tables.
If true, a #GArrowField of #GArrowNullDataType can be unified with a #GArrowField of another type. The unified field will be of the other type and become nullable. Nullability will be promoted to the looser option (nullable if one is not nullable).