For a Synology NAS, the reset method depends on what you want to do:
Mode 1
According to the knowledge base, Mode 1:
- restores the admin account password and network settings to default
- does not erase the data on the NAS
- for some models/docs, it is described as resetting the IP address, DNS server, and admin password to default
Mode 2
According to the knowledge base, Mode 2:
- removes all system configurations
- re-installs DSM
- does not erase the data saved on the NAS during reinstallation, based on the DSM 4.3 guide
- in the older DSM 4.3 guide, restoring factory defaults via the software option can format the hard disk and erase all user data, but the manual RESET-button reinstallation process says the data will not be erased
Reset procedure mentioned in the sources
Summary
If you want, I can also summarize the differences between the Control Panel > Restore Defaults options and the RESET button procedure.
Sources used: [1], [9]