RFSP index refers to "RAT/Frequency Selection Priority (RFSP) index". It is a parameter used to configure the cell re-selection and cell selection behaviour of a mobile device.

RFSP index is a key parameter used by mobile networks to control how mobile devices select and re-select cells, with the goal of steering traffic onto the operator's preferred RATs and frequency layers.

A higher RFSP index value indicates a higher priority for that particular RAT/frequency combination.
The RFSP index is signalled to the mobile device by the network, as part of the system information broadcast.

For example, the network may assign a higher RFSP index to the LTE layer to encourage mobile devices to prefer LTE over 3G or 2G, if that is the operator's preferred RAT. Or it may prioritise one LTE frequency band over another to balance load across the network.