RE: What does phase MAIN_TRANSEXEC/DDIC_TRANS do in SUM
I suppose that DDIC_TRANS get similar activities as DDIC_UPG.
The phase DDIC_TRANS is there when you choose single instance option for the update/upgrade.
DDIC_UPG will be there is you choose standard option instead.
And here is activities performed in this phase.
DDIC import of all SAP packages and SPDD