Why are those Sign-in/sign-out icon a convention?

1. What are the 3 types of icon?
2. What is the difference between a symbol and an icon?

What are the 3 types of icon?

There are three types of icons: “universal,” “conflicting” and unique icons. Let's focus on each type and its impact on the user experience.

What is the difference between a symbol and an icon?

“Icon” and “symbol” are terms that we often use interchangeably, but there's actually a subtle difference: An icon is a literal, visual representation of the thing it represents, whereas a symbol can be a more abstract representation of something.