Pen emoji is a picture of an ordinary ballpoint Pen of blue or black color. Such pens are commonly used everywhere, where it is necessary to write something down, along with the item represented by ✏️ Pencil emoji. So no wonder that it is used in the context of offices, schools, colleges, and any other establishments, or just as a symbol of taking notes of signing up something.
Examples of using
“I should have written that down 🖊”
“I always have a pen by me 🖊”