Difference between page and env

What is the difference between page and env objects given the code below?
I cannot find an explanation why sometimes `page` is blank. What are best practices for getting a uniqueId for a message on all pages, including Print.

#if env.context.message??
  #assign msg_id = env.context.message.uniqueId
#else
  #assign msg_id = page.context.message.uniqueId
#if

Thank you,