Level
Grid data is presented via levels. In tabular Views, if data is not grouped by any grouping item, it is displayed at level 0. If you group data, nesting levels are created. To display data at a specific level, you must click the [+] button. Thus a level can be defined as a number of [+] buttons you need to click to reach the desired record.
If you group data by two grouping items, three levels are created.
In Chart Views, levels correspond to data levels.