Minecraft wiki entry for NPC, associated text: Advanced Settings
The advanced NPC settings consists of URL and commands. They are used to create interactive buttons. The current maximum advanced settings that can be added is 6.
URL‌[Education Edition only]
The URL feature allows the player to add a hyperlink button. The URL can be entered at the first input box below the title up to 1024 letters.
To change the button name or label can be proceeded by filling the input box titled "Button Name", the current text limit for the label is 16 letters.
When added, the button appears only if the player has the world builder permission disabled. The button label defaults to "Learn More" and the color defaults to blue, it can be changed by using the color codes.
Command
The command feature allows the player to add one or more executable commands in the NPC. The command can be entered at the first input box below the title, unlike the command block wherein multiple commands can be executed in one window. The current maximum characters for the command is 1024. The command(s) get executed when the dialogue with the NPC is closed.
The command can be presented either as a button or a direct command. As a button, the button name or label can be changed by filling the input box titled "Button Name", the text limit for the label is 16 letters. It can be executed only if the player has the world builder permission disabled.
When not added as a button, the command is executed after the dialogue with the NPC, visible by right clicking the NPC, has closed. In contrariwise, the button is displayed and has a label color defaults to light gray, and can be changed by using the color codes. (the default name of the NPC is NPC)
@initiator‌[Bedrock Edition only] is a specialized Target selector that targets the player that has interacted with the NPC, distinguishing them from other players that may interact later. This is useful for running commands on a per-player basis.