What is the subject (mubtada')?
A جُمْلَة اِسْمِيَّة (jumlah ismiyyah) opens with an ism, and that opening ism is the مُبْتَدَأ. It is what the sentence is about. What is then said about it is the خَبَر.
In مُحَمَّدٌ رَسُولٌ (muḥammadun rasūlun), Muhammad is a messenger, مُحَمَّدٌ is the mubtada' and رَسُولٌ is the khabar. Both carry rafa. Both must.
The mubtada' is what you are talking about; the khabar is what you say about it. Both are marfū'.
Notice what Arabic does not need. English cannot say “Muhammad a messenger” — it must insert is. Arabic completes the sentence in two words, with no verb at all, and the two ḍammas hold it together.
The mubtada' is defined by what stands before it: nothing. The moment a governing word such as إِنَّ or كَانَ enters, the word is no longer called a mubtada' — it becomes the ism of that governor, and may no longer be in rafa.
Like most Arabic grammar terms, مُبْتَدَأ (mubtada') was taken from everyday speech and then given one precise technical sense.
In everyday Arabic it means a thing begun, a starting point.
اِبْتِدَاءُ الْعَامِ (ibtidā'u al-'āmi) — “the beginning of the year”
In Arabic grammar it means the ism a nominal sentence begins with. Do not confuse it with مُبْتَدِئ (mubtadi'), which means a beginner.