Causal clauses are subordinate clauses that give the reason for or cause of something.
They start with the conjunctions "weil" or "da," which both mean "because."
The question words are: "Warum?" "Wieso?" "Weshalb?" "Aus welchem Grund?" "Weswegen?"
A: „Warum bist du 1 Stunde zu spät?“
B: „Weil ich den Bus verpasst habe.“
When the main clause comes first, we usually use "weil."
When the subordinate clause comes first, we usually use "da."
"Weil" and "da" have the exact same meaning and there is no difference in terms of grammar. You can use whichever you'd like in any situation.
It's just a suggestion that you use "da" at the beginning of a sentence and "weil" in the middle since it's more common.
A subordinate clause starting with "weil" can also stand alone without a main clause, as long as it's obvious from the context what the other half of the sentence would be.