explain this custom function, please?

Hi halifaxious,

There is a lot going on here

replacer = (x,y,z)=>LookupTable{[#"lookup value"=x]}?[replacement]? ??x

First this is a custom replacer function which takes 3 arguments: 

x = original value

y = match condition (true or false)

z = replacement value

Next its applying (optional) key match lookup to identify a unique row in the table

LookupTable{ [#"lookup value"=x] }?

Followed by (optional) field access, to return a value from a field/ column

[replacement]?

And finally applying coalesce, to return the input value if a null is returned.

?? x

Ps. If this helps solve your query please mark this post as Solution, thanks!

Some links to resources on these topics: 

Visual guide to key match lookup 

Visual guide to coalesce 

Item- and Field access