kernel – separate Bitrix24 product kernel in a new site directory. Contains database, site files, web service configs.
ext_kernel – separate Bitrix24 product kernel in the new site directory for creating links to this kernel within multiple sites. Kernel will become inaccessible directly and only via additional sites (operates jointly with link type sites). Contains database, site files.
link – in case of creating additional site within multiple sites structure. Common kernel and data in the general database with already pre-installed Bitrix24 product (operates jointly with the ext_kernel or kernel cores). Contains web service files (but not all) and configs. Subdirectories bitrix, upload have links to ext_kernel or kernel.