RANK and INDEX are part of Table calculations in Tableau.
INDEX mainly deals with physical position of the record, incremental numbers are assigned based on the physical order of the records.
Example:
| SUM(SALES) | INDEX |
| 10000 | 1 |
| 40000 | 2 |
| 20000 | 3 |
| 30000 | 4 |
RANK mainly deals with the value of the record, highest value gets highest rank, lowest gets the lowest rank.
Example:
| SUM(SALES) | RANK |
| 10000 | 4 |
| 40000 | 1 |
| 20000 | 3 |
| 30000 | 2 |