What is the source of this mathematical rubbish?
Are you omitting some clarification?
In the material I'm studying, it says the following;
. . 1. A number sequence is when addition is written horizontally.
. . 2. A number sentence is when subtraction and multiplication are written horizontally.
Is this really specific to addition, subtraction and multiplication, or can they all be interchanged?
Could I say that, 2 + 4 = 6 is a number sentence and that 4 x 5 = 20 is a number sequence?
Statement 1 makes no sense.
A number sequence is an ordered list of numbers.
Example: 2, 5, 8, 11, 14, ...
. . which may or may not have a pattern.
We see that: . is not a number sequence . . . and neither is:
Who cares if it is written horizontally or vertically?
Statement 2 might make sense if we knew what a "number sentence" is.
Perhaps it is an English sentence containing a number?
Example: "There are twelve objects in a dozen."
I seriously recommend you discard that material immediately.
It is chillingly irrational and dangerously wrong.