what is app name's PLACEHOLDER in spring boot properties file

In application.properties file use the following PLACEHOLDER

1.
app.name = any_name_you_want_for_your_app

2
How do you set description PLACEHOLDER in application.properties file
app.description = ${app.name} is a Spring Boot application 

3. How do you set the server context path in application.properties
server.context-path=/${app.name}

4. How do you set port in application.properties file?
server.port = 9091

5. How do you configure external file path in application.properties file?
prefix.jsonFileLocation = /home/ishaan/docs

Placeholder ref