‘Posting’ in the pen world refers to what you do with your pen cap while you write. Fountain pens, with the exception of a few new pens like the Lamy Dialog 3 (below), are two-part pens: a body and a cap.
Posting is when you put the cap on the back of the pen while you write, regardless of whether it pushes on or screws on with threads.
The cap unscrews from the pen, and you write with the body, but what do you do with the cap? Do you put it on the desk? Hold it in your hand? Both of these would be ‘non-posting’ or ‘unposted’ writing methods.
I’ve been told (by no scientific method of discovery) that most Americans post their pens, whereas most Europeans hold the cap in their non-writing hand. Just pulling a theory out of the air, I suspect the difference is caused by our writing culture. Europeans (in general) are raised with a familiarity of fountain pen usage, and holding the cap in your hand is a method of security…should someone ask to borrow your pen, they won’t be likely to walk off with it if you still have the cap! I suspect most Americans, if they are using a fountain pen, will not likely be asked to have their pen borrowed. Most of the people I meet that see me using a fountain pen in public hardly know what it is, let alone how to use it. No one has ever asked me to borrow one, unless it’s a fellow fountain pen users that I know.
So is one way right and one way wrong? Heck no! Do what you want. Depending on the weight and balance of the pen and your own personal preference, you may have a strict way of posting or you may be flexible from one pen to the next. Me? I’m a non-poster. I like have more weight on the nib of the pen, and posting the cap throws my balance off. What do you like to do?