Seal, mark, and sign are all synonymous.
The mark is a sign that you worship the beast.
And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand.
- Revelation 14:9 (KJV)
The sabbath is a sign that you worship God.
Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant. It is a sign between me and the children of Israel for ever: for in six days the Lord made heaven and earth, and on the seventh day he rested, and was refreshed.
- Exodus 31:16-17 (KJV)
The Sabbath is clearly a set day for worship, and is part of the Ten Commandments.
Currently there are only two worship days that compete with each other.
If you keep the Sabbath (Seal of God), it is a sign that you worship God.
If you keep the Sunday (Mark of the beast), it is a sign that you worship satan.