Both in list and in data table, there are key columns. List can only have one and table can have no key columns or any number of them.
In key column every value must be unique - there can't be two cells in it with the same value. For example in list SYSWORKSPACE key column is ID - there can't be any value repeated, but in other columns they can, like in COLOR.
When in table there is more than one key column, in a single column there can be the same values, but two rows can't have identical values in all key columns. For example, in the table below, two rows can't have the same year, month and day.
Remarks - action step save to database:
When using save to database action step, you have to pay special attention to where the key columns are. If you want to save a row, in which key column has a value which was already in database, this row will be updated - not added as a new one.