What is the use of buffer and env variable in the inject node,where can I use them
They are other types of data you can inject into the flow.
The Buffer type let's you inject a node.js Buffer object - typically used to inject binary data.
The Env type let's you inject the value of an environment variable.
It's up to you when to use them, and whether you have a particular need for them. The Env var option can be useful where you run the same flow on multiple devices and want to customise some value per-device without having to change the flow.