Please go through the definitions mentioned below. I hope they will clarify your doubts. If not, please see the attached PDF, which I extracted from the "Process Safety" document.
Severity Rate
The severity rate is a calculation that gives a company an average of the number of lost days per recordable incident. Please note that very few companies use the severity rate as a calculation, as it only provides an average. The calculation is made by dividing the total number of lost workdays by the total number of recordable incidents.
Severity Index
The severity index calculations include a category for "Community/Environmental" impact and a first aid (i.e., OSHA "recordable injury") level of Safety/Human Health impact, which are not included in the PSI threshold criteria. However, the purpose of including both of these values is to achieve greater differentiation of severity points for incidents that result in any form of injury, community, or environmental impacts.
I hope this helps. If I am wrong, please educate me.
Thanks.
Regards