IPD is treatment where the patient is formally admitted (surgery, serious illness, childbirth). This is what standard GMC is built to cover - room, procedures, ICU, surgeon fees. It's lower-frequency but high-severity, which is exactly why it's insured. The complement to IPD is OPD, the everyday care outside admission.
In India
The OPD/IPD distinction is the single most-searched HR insurance term pair. Rule of thumb: admitted overnight = IPD; walk in, walk out = OPD.
In practice
A knee replacement requiring a 3-day admission is an IPD claim under GMC. The follow-up physiotherapy and painkillers afterwards are OPD - covered only if the employee has an OPD benefit.