Hello Giovanni
IPv6 source guard is a layer 2 snooping feature that blocks any traffic from an unknown source. An unknown source is an IPv6 address that is not already in the binding table or has not previously been learned through ND, as described in the lesson.
IPv6 destination guard will ensure that a device performs address resolution only for those addresses that are known to be active on the link. It uses what is known as address glean functionality. Address gleaning involves snooping Neighbor Discovery Protocol (NDP) and DHCP messages on the link to populate the binding table. When a packet reaches the device and there is not yet an adjacency for the destination or for the next hop, the NDP consults the device binding table to verify that the destination or the next hop has been previously gleaned. If the destination is not found in the binding table, the packet is dropped. Otherwise, neighbor discovery resolution is performed.
I hope this has been helpful!
Laz