How does 4 digit addends have a 5 digit sum

A 4 digit addends could have a 5 digit sum because of the rule of regrouping.

Regrouping is a situation where carrying and borrowing is involves. So let’s have an example

8999

+5678

14677

so in this example, we added 8999 to 5678.

9+8 = 17, so we bring down 7 and carry 1

9 + 7 = 16 + 1 = 17, so again we bring down 7 carry 1

9 + 6 = 15 + 1 = 16, bring down 6 carry 1

8+5 = 13 + 1 = 14, since this is our last number, bring down 14 ...

=> 14, 677