Economies of scale differ from learning-by-doing in that the former concept concerns cerrent production and the latter cumulative production over the products' life cycle. In (3), we are only interested in economies of scale that are increasing in units produced,i.e. that costs are continuously reduce when production increases at a given time. Economies of scale that are being utilised only once when a given limit in production volume is passed, will only shift the learning curve (change the level) and not affect the PR.
The technology specific variable can be used to handle circumstances that are specific to the actual technilofy. For instance it can be the average wind speed for wind energy