Hi, Sara
I cannot confirm the meaning of a slice but the SRX's boot media is divided in different slices (s1, s2, s3, s4) and these slices are divided in different partitions as you stated.
The SRX will boot up from specific partitions in s1 and s2, usually partition "a".
Each slice is partitioned in the following way:
root@SRX-8# run show system storage partitions
Boot Media: internal (da0) <<< Nand flash
Active Partition: da0s1a
Backup Partition: da0s2a
Currently booted from: active (da0s1a) <<<<< *
Partitions information:
Partition Size Mountpoint
s1a 610M /
s2a 617M altroot
s3e 46M /config
s3f 618M /var
s4a 56M recovery
s4e 5.7M
* whichever partition is the active one it is known as the root partition, and the one that is not active is called the alternate partition.
Also you can see the Junos version on each parition with the following command:
esteban@vipfw> show system snapshot media internal
Information for snapshot on internal (/dev/ad0s1a) (primary)
Creation date: Aug 17 15:28:15 2018
JUNOS version on snapshot:
junos : 12.3X48-D50.6-domestic
Information for snapshot on internal (/dev/ad0s2a) (backup)
Creation date: Jan 24 16:46:31 2018
JUNOS version on snapshot:
junos : 12.3X48-D50.6-domestic
Hopes this helps.