What NSSF actually does
NSSF (National Social Security Fund) is Kenya's mandatory pension scheme. Employee contributes 6% of pensionable pay, employer matches another 6%, both go to your retirement corpus.
Tier I vs Tier II
Tier I covers the first KES 8,000 of your pay. Both employee and employer contribute 6% of that (so KES 480 each). Tier II covers earnings between 8k and 108k, again 6% each. Above 108k, no further NSSF, that's the upper earnings limit.
Can I voluntarily contribute more?
Yes. Anything you put in above the mandatory minimum (up to KES 30,000/month combined with private pension) is deductible from your taxable income before PAYE. If you're employed and paying tax, this is one of the most tax-efficient long-term moves you can make.