what is service url in spring starter project? Alternatives for default service url https://start.spring.io

I am new to Spring boot.
While creating a New Spring Starter project in STS there is a Service URL dropdown defaults to https://start.spring.io as given below.

I searched in stack overflow but didn't find any information about it. What is the use of specifying this? Why it has been given as editable? Is there any alternative available instead of https://start.spring.io?

I noticed that tried with any other URL, it tries to parse as json and gives exception given below.

JSONException: A JSONObject text must begin with '{' at character 3

Need clarification, what is happening in background?

It is a simple client server architecture which various UI clients such as Spring Boot CLI , IDE or official web UI talk to the same backend HTTP API that actually generates the spring starter project.

The service URL is the URL of this HTTP API. Spring Team already deploy an official one at https://start.spring.io which you can simply use it.

But in case if you want to build your own customised version , you can checkout its source codes , modify according to your needs and deployed to your own server. Then change the service URL to your API server 's URL in order to use it.