This page explains some common abbreviations and terms:

CA - Control Agent. That's a separate module that talks with Kea DHCPv4, DHCPv6 (and soon also D2) over control channel and exposes their internal commands using RESTful interface.
D2 - This is a nickname of DHCP-Dynamic DNS server module. Since using the full name is awkward, we often use shortened version of it: D2.
DHCP - Dynamic Host Configuration Protocol. There are two similar, but operationally different protocols: DHCPv4 and DHCPv6. When v4 or v6 is not specified, the DHCP expression applies to both.
DHCPv4 - Dynamic Host Configuration Protocol for IPv4, a protocol that defines how IPv4 hosts can obtain IPv4 addresses and other configuration from the servers. Defined in RFC2131.
DHCPv6 - Dynamic Host Configuration Protocol for IPv6, a protocol that defines how IPv6 hosts and router can obtain IPv6 addresses, IPv6 prefixes and other configuration from the servers. Defined in RFC3315.