1. I/O type is specified in IC design spec by customer. Type is defined by periphery, that should be connected to this I/O.
For 2.5V supply voltage exist a lot of I/O types, e.g. LVCMOS, LVTTL, ... See correspondent JEDEC standards for clear definition. It should be noted, that IC core and periphery supply voltages could be different, and suitable I/O subset, incorporating level converters, should used.
2. Customer and/or IC designer should define a concrete I/O type.