These two fields apply only to Hashed tables. tablesONLINE allocates space for a Hashed table so that the specified number of rows results in the specified Lower Density field. For example, if 50 percent lower density is specified with a 100-row table, the space allocated for the table is for 200 rows.
When the density of a table reaches the specified upper density, space is allocated to bring the table to the specified lower density. For example, if 80 percent upper density and 50 percent lower density are specified for a 200-row table, when the table reaches 160 rows, it is expanded to have space for 320 rows.