For Dlink discoveries, a device will appear as Modified if it has already been authorized in PulseNET, but has returned an IP or Port value that differs from the existing record. Selecting these, and accepting the modification will overwrite the existing record with the latest that was retrieved.
For SNMP, any device that has been discovered previously will list as a Modified device if included in the discovery range. So when getting Modified returns on an SNMP discovery, it's effectively just a: "Yes, we still see this device online in the range you selected, but there's no reason to authorize it again." Just an active record of the already authorized devices from the last range attempt.