Seeking knowledge on an unknown VBA concept

Disclaimer: "unknown" in Thread title means that I don't know the name of the concept I am about to describe. (Have not done a VBA course yet so please forgive what is probably a newbie question).


Please see pseudo code below

Sub Test1()
dim a as string
dim b as long
a = "xyz"
b = 1.45
(some code here)
End Sub

Sub Test2()
(some code here)
dim a as string
a = "xyz"
(some code that uses a)
dim b as long
b = 1.45
(some code that uses b)
End Sub

Assume that the above two sketches are the same macro except that in the first I am declaring my Dims etc as soon as the macro starts and in the second I am declaring 'just in time' (so they are declared just before the code lines that require them).

1. What is the name of the concept I have described above?
2. What is the pros & cons of each method?
3. Would the second method be faster as the memory doesn't have to hold the data when it doesn't need to?

Appreciate any answers to the above!