The SDA is the WTS’s variant from a branched timeline.
The term "Second Adventist" is a broader term encompassing various groups that share a belief in the Second Coming, while "Seventh-day Adventist" specifically refers to a denomination that observes Saturday as the Sabbath, also believing in the imminent Second Coming. Second Adventists (like Seventh-day Adventists) believe in an imminent, literal return of Christ in the flesh, while Russell and Jehovah's Witnesses believe in an invisible, spiritual return. Second Adventists generally believe in the divinity of Christ, including His pre-existence and role in creation, while Russell's (Bible Students, name changed to Jehovah’s Witnesses in 1931 by Rutherford; Bible Students do not consider themselves as Jehovah’s witnesses at any time) Jehovah's Witnesses teach that Christ was created by God and is not a deity. Second Adventists generally believe in the Trinity, while Jehovah's Witnesses believe in the supremacy of God and reject the Trinity. (Charles Taze Russell was never part of a Second Adventist group though he read some of their teachings and discuss teachings with members, eventually left for some basic doctrinal differences as outlined above)