The objects are placed based on sequential order in a sequence diagram.

The vertical dashed lines are the "lifeline" that shows the existence of the objects or actors over time; moving down the lifeline means more time is passing.

"Request PIN" is a message, and it is not a reply message; because "Request PIN" is not replying to an earlier message and is requesting information rather than responding

An alternative Frame symbolizes a choice between two or more message sequences.

"PIN entered" is a response to the "Request PIN", so "PIN entered" is a reply message $($dashed line$).$

"Activation Box" shows when and how long an object is active $($performing a process$)$ or idle.