Hi Tibor,
basically, hashmarks are used when you want to subsitute some values dynamically.
Let's say you have a local variable named Temp whose value is section ID which changes depending on employee ID.
Now you want to pass this to a query, you cannot obviously pass the ID but assign it to Local.Temp and Pass '#Local.Temp#' to it to substitute the value dynamically.
So the query is called with different Temp values generated dynamically depending on some logic.
Hope this helps.
Best Regards,
Swaroop