The Metz 45 Digital is needed for its preflash capability which is needed to support digital camera TTL (iTTL, dTTL, eTTL, etc) which the older Metz 45 cannot do.

The Metz 3000C is an adapter which allows the use of an appropriate Metz SCA 3000 module (3102 for Canon, 3402 for Nikon). The 3000C is not used with SCA 300 units, like the 390 for Hassy TTL bodies, the 356 for Bronica TTL bpodies.