Question on ISTag in ICAP protocol

Hi,

I am reading section 4.7 regarding ISTag in ICAP
response. In the section, it mentioned that the change
of ISTag should invalidate all entities generated by a
particular service. Could someone clarify for me
whether this "entities" referring to objects in "ICAP
CACHE" (cache to store ICAP responses), or objects in
"HTTP CACHE" (in the case that the ICAP client is a
HTTP proxy. 

Thanks!
Yimin 

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