SMC-S applies when you have a single 100% rating and an additional unrelated 60%+ rating, or you are permanently housebound by service disabilities. 38 CFR § 3.350(i) defines the criteria. This replaces your 100% rate. TDIU counts as 100% for the 100+60 rule.
Two paths: (a) 100% + 60% from different disability groups. (b) Housebound: nearly confined to home. Define housebound per 38 CFR 3.350(i). SMC-S is a compensation benefit, not pension.
For 100%+60%: VA ratings table to verify separate conditions. C&P exams. For housebound: physician's statement, VA Form 21-2680, or Social Security/agency disability determinations showing home confinement.