Calligraphy pen, round pen must hide the front, a typical example is the Zheng Wengong tablet, or you can see some modern Zheng Wengong tablets copied by everyone, which is more intuitive.
On the other hand, the hidden front is not necessarily a round pen. Look at Liu Gongquan's handwriting. Are all the pens hidden? This is typical of Fang Bi. Of course, on the whole, Liu Ti is used by Fiona Fang.