LDC stands for Lower Division Clerk and DEO stands for Data Entry Operator — both recruited through SSC CHSL.
LDC work is general clerical work: files, noting, drafting and correspondence.
DEO work is focused on data entry and computer-based processing.
DEO posts require a higher data entry speed in the skill test than LDC.
Pay levels of the two posts can differ depending on the office.
The official CHSL notification lists which vacancies are for LDC and which are for DEO.