The desc explicitly states this is the count of IPsec bytes non compressible — outbound traffic the compression algorithm determined wasn't worth compressing, such as data that is already compressed, like encrypted files or media. A high value relative to cpvIpsecComprBytesBefore tells you this tunnel is mostly carrying traffic that gains nothing from IPComp, such as already-compressed backups, images, or video, useful evidence that compression isn't actually helping on that particular tunnel and could be disabled. e.g. cpvIpsecNonCompressibleBytes making up the bulk of traffic on a tunnel primarily used for offsite backup transfers of already-compressed archive files.