A Reporter Block is a block that contains a value. These blocks can contain anything, from numbers to sentences.
As Reporter blocks are values, they are designed to fit wherever a value is needed. This can be anywhere: a lot of blocks have spaces in them for input, or Reporter blocks. An example:
This block can easily be filled with Reporter blocks:
File:Glide (Costume -) Secs to X (X) Y (Y).png
As Reporter Blocks are values, they are used whenever a script needs a certain value. These values can be anything - from X positions to the costume number of a sprite.
An example is below:
File:An example script for the use of Reporter blocks.png
Note how the Reporter block is used in the script - the script waits until the Reporter block's value reaches a certain point, and then it continues.