The router’s NVRAM used to contain the startup configuration file only, but this is no longer strictly the case. Recent IOS releases also use the same NVRAM space to store information such as private keys for SSH or IPSec (private-config), and interface numbers for SNMP (ifIndex-table).
Directory of nvram:/
20 -rw- 5068 <no date> startup-config
21 ---- 2302 <no date> private-config
1 ---- 0 <no date> persistent-data
2 -rw- 133 <no date> ifIndex-table
You cannot delete nor read these two files as they strictly belong to the IOS system on your switch or router. Those files do not contain anything sensitive. It is strictly system information that has no relation to any configuration.
Update: OK so i've been searching all over for this and i found this thread:
There's an error generating the following message:
%PRST_IFS-3-GENERAL: persistent media IFS general error: Open Read - can't open nvram:persistent-data errno is 16
Searching for this lead me to believe that the Cisco IOS File System (IFS) saves some persistent date (variables) about the Cisco image in the NVRAM. This data is used upon booting the switch or router.