I have a stock module with two fields one Stock Re-order level and one Item Movement. Based on the item sold i want the item movement to be changed automatically for example: ‘Item Movement’ (e.g. Best-seller, Fast, slow etc.) should be based on conditional  formula per product item and linked to 'Stock re-order level’. For example, if product A reaches ‘Stock re-order level’ in less than one month, it is categorised as ‘Best Seller'’. If Product A reaches ‘Stock re-order level’ in 2 months, it is categorised as ‘Fast’. If Product A reaches ‘Stock re-order level’ after more than 2 months, it is deemed ‘slow’. Is there any possibility to automate the process?

Doesn't sound that difficult but we need some insight into where your data is, what tables you have and what you have tried already.

This is a typical thing where you would not want this to be a calculation but a routine that runs once a day to reclassify all products.