LK1,LK2,LK3 Manufacturer ID
LK1,LK2,LK3 connect to PPI Port B, Bit1-3. The links select the manufacturer name (which is displayed by the BIOS in the power-up message). These LKs exist on all CPC mainboards (in CPC Plus/GX4000 schematics they are named LK101,LK102,LK103).
LK4 Screen Refresh Rate
LK4 connects to PPI Port B, Bit4. The link tells the BIOS whether it should set-up the CRTC for 50Hz or 60Hz refresh rate.
CPC6128 : LK5,LK6,LK8
The CPC6128 contains 128K RAM, RAM banking is controlled by a PAL (which assists its Gate Array). However, the 6128 mainboard can be also used for 64K computers - in that case, only 64K would be installed, the PAL would be missing, and LK5,LK6,LK8 would be installed (bypassing the RAM banking logic, and passing signals directly to unbanked RAM).
DDI-1 : LK1
In the DDI-1, LK1 connects both to the Expansion Port /EXP pin, and to the AMSDOS ROM bank number selection logic. The latter one allow to map AMSDOS to bank 7 (default), or bank 0. If AMSDOS sees itself mapped to bank 0, then it does automatically load & start the boot sector from drive A (equivalent as when typing |CPM after power-up).
DDI-1 : LK2,LK3
In DDI-1, LK2-LK3 connect to the Disc Data Separator. Purpose...?