Hello Sakshi,
INTB is an open-drain active-low output with internal pull up (10kΩ) to VDDIO. When there is no fault or event, INTB is high (via this internal pull-up). When FS85 asserts an interrupt, INTB is pulled low. Each interrupt can be masked by setting the corresponding inhibit interrupt bit in M_INT_MASK registers.
On the MCU side, you should configure the GPIO connected to INTB as an input with interrupt capability on falling edge. In an interrupt handler, you should read M_FLAG registers over SPI to determine the cause and de-assert the interrupt pin.
BRs, Tomas
