The public_transport key denotes stop positions and platforms of public transport. Stop position are placed as node on the street. Platforms are placed next to the street. Both are members of the relation stop_area and route relations.
Guide for bus stop tagging in 60 seconds
Stop position
A 
node, on the way which the bus travels on, at the position where the bus stops:
Platform
An element ( node, 
way, or 
area) where the passengers wait:
Stop area
A 
relation including all stop positions (role: 
stop
Values
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