What is the difference between pin-to-pin compatibility and drop-in compatibility?

Description

Altera refers to drop-in compatibility as vertical migration.

Pin-to-pin compatibility occurs when the power pins, ground pins, and dedicated pins are at the same pin location across device densities. An example of a pin-to-pin compatible device would be the EPM7128AETC100 or EPM7128BTC100 device.

Drop-in compatibility occurs when the devices operate at the same voltage level and they are pin-to-pin compatible. An example of a drop-in compatible device would be an EPF10K20RC208 or EPF10K30RC208 device since they require the same supply voltage of 5.0 V.