You have known the difference between UNION and UNION ALL. But there is similarity when used sequence. It is not possible to use them to combine sequence values.
Create the following sequence
CREATE SEQUENCE MYSEQ AS INT START WITH 1 INCREMENT BY 1
You get the sequence value using
SELECT NEXT VALUE FOR MYSEQ
If you want to use UNION or UNION ALL combing different sequence values, it is not possible. Both the following codes when executed throws error
SELECT NEXT VALUE FOR MYSEQ UNION SELECT NEXT VALUE FOR MYSEQ SELECT NEXT VALUE FOR MYSEQ UNION ALL SELECT NEXT VALUE FOR MYSEQ
There error is
Msg 11721, Level 15, State 1, Line 2 NEXT VALUE FOR function cannot be used directly in a statement that uses a DISTINCT, UNION, UNION ALL, EXCEPT or INTERSECT operator.