Form plugin code -- getMetaConfig(): Promise NintexPlugin versus Promise PluginContract

I started a form plugin C# / Lit project using the Nintex Form Plugin Starter project available at https://github.com/NTX-XT/ntx-form-plugin-starter

and it has this line,

export class PageHighlight extends LitElement {
    static getMetaConfig = (): Promise<PluginContract> | PluginContract => config;

and I used it to build a web component that allows a user to select a date from the calendar. 

I now want to store the selected date for use in the form or workflow, so I am trying to follow the example code at https://help.nintex.com/en-US/formplugins/Examples/CreatePlugin_MaterialTextField.htm

which tells me to insert this line,

static getMetaConfig(): Promise<NintexPlugin> | NintexPlugin

As you can see, it uses Promise<NintexPlugin> instead of Promise<PluginContract>.

So which one do I use, and what is the difference between them?

Where is the documentation on how to use getMetaConfig?