All of these statistics employ the access log fields called cs-bytes and sc-bytes.
The access log field- cs-bytes- declares the bytes transmitted from the Client to the Secure Gateway (SG), which is the statistic Bytes uploaded. The access log field- sc-bytes- declares the bytes transmitted from the SG to the Client, which is the statistic Bytes downloaded. Total Bytes is simply the two fields added together for any one user/client IP pair.
The Cache Bytes statistic employs one more field in the access log to determine if the bytes received was cached or not; the s-action field. If this field contains the text "hit" anywhere, such as TCP_HIT, the bytes downloaded for any user/client IP pair will be added up, as each row in the access log is processed, and declared as "Cache Bytes". The Cache bytes statistic can be used to compare the relative gains made, because of Cached data, on each user/client ip pair.
NOTE1: For complete list of what fields are expected in the SG access log , see 000021974