Does "yield" cast lambdas to procs?

Question

I'm learning Ruby, and while testing the behaviour of procs and lambdas, I wrote this code:

def test_f(&a_block)
  puts "in test_f"
  yield 7
  puts "still in test_f"
end

def test_f_2(&a_block)
  puts "in test_f_2"
  a_block.call(7)
  puts "still in test_f_2"
end

lam = lambda do |i|
  puts "lambda #{i}"
  return "lambda #{i}"
end

test_f_2(&lam)
test_f(&lam)

The result of running this is:

in test_f_2
lambda 7
still in test_f_2
in test_f
lambda 7
test.rb:17: in 'block in <main>': unexpected return <LocalJumpError>
        from test.rb:3:in 'test_f'
        from poop.rb:21:in '<main>'

I have two questions from this:

1. Does this mean that using yield 'casts' the lambda into a proc, since it is only when using yield that attempting to return causes an error?
2. Under what circumstances is it not permitted to return from a proc?

I've read that "it's not possible to call a proc containing a return when the creating context no longer exists" but the context here is global so it doesn't make sense to me to say that the creating context for lam no longer exists. However, enclosing the entire creating of lam and calls of the test functions within another function prevents any error message from appearing, which leads me to conclude that the criterion at play is that any call to a proc containing a return must be from within a context from which you can return.

Am I right?