(1) is there a definition for "station spreading"?
Offically no, we define it as increasing coverage by extending a staion unnecessarly or adding station parts that are unused. See ST2's examples
(2) what is a purpose of station spread limit? Doesn't it limit the spread?
Station spread limits the size of a railway station, it does not limit the spread of adjoining vehicle stations.
(3) what is a purpose of CHIP tiles, besides spreading?
they are decorative
(4) why CHIP tiles are allowed?
Why should they not be?
"well, forum search clearly says: there is NO definition for station spreading." - try reading the server rules particually No.7
Rule #7
Do not station spread with airports, train and road vehicle stations to extend coverage area to additional resources including towns and cities unfairly.
dodger007