From NAV 2015 onwards, Microsoft has change function's default behavior as local.
Normally in general coding, it is best practice to declare a function as a local function where only within the scope this function can be access. However with earlier versions of Microsoft Dynamics NAV, Microsoft has set this to global, That means from anywhere you can call the functions.
Function's default behavior is based on codeunit Subtype
If the subtype is,
There will not be any impact when you will upgrade from NAV 2016 to NAV 2018.