The HSI specification is currently incomplete and in active development.
Sample output for my Lenovo P50 Laptop:
Host Security ID: HSI:2+UA!
HSI-1
✔ UEFI dbx: OK
✔ TPM: v2.0
✔ SPI: Write disabled
✔ SPI: Lock enabled
✔ SPI: SMM required
✔ UEFI Secure Boot: Enabled
HSI-2
✔ TPM Reconstruction: Matched PCR0 reading
HSI-3
✘ Linux Kernel S3 Sleep: Deep sleep available
HSI-4
✘ Intel CET: Unavailable
Runtime Suffix -U
✔ Firmware Updates: Newest release is 8 months old
Runtime Suffix -A
✔ Firmware Attestation: OK
Runtime Suffix -!
✔ fwupd plugins: OK
✔ Linux Kernel: OK
✔ Linux Kernel: Locked down
✘ Linux Swap: Not encrypted