The ID of a block must be unique in the project.
When referencing a block in an equation by name, Ecolego first looks in the sub-system of the block whose equation you edit. If it there is no block with that name in the sub-system, the root sub-system is searched. If no block is found there either, an error message is presented.
If you want to explicitly reference a block in another sub-system you must use the ID of the block. For instance, using the hierarchy above, if you want to create an expression in the sub-system France which uses the parameter Price, you must write the full ID of the parameter:
TotalPrice = France.Paris.Price + France.Orleans.Price + ...