This is arc_c from the ZFS ARC. This shows the target size that the ARC is attempting to maintain. For the actual size, see the Advanced Analytic Cache: ARC Size.
An understanding of ZFS ARC internals may be required to properly interpret this statistic.
When to Check ARC Target Size
Rarely; this may be useful for identifying internal behavior of the ARC, however there are other statistics to check before this one.
If there are caching issues on the appliance, check the Cache ARC accesses statistic to see how well the ARC is performing, and the Protocol statistics to understand the requested workload. Then, check the Advanced Analytics Cache ARC size and Cache ARC evicted bytes for further details on the ARC behavior.
ARC Target Size Breakdowns