Baking – How much lemon extract to substitute anise extract

Imitation vanilla extract is meant to be used in place of the real stuff, and most recipes probably assume you're using the fake stuff anyway, so you would use 1 tsp. You only need to adjust the amount if you're using something like twofold (2x) extract.

A flavoring extract is flavoring disolved in alcohol, while a flavoring emulsion is flavoring suspended in water with an emulsifier. Citrus oils like lemon have a stronger flavor when placed in an emulsion than an extract, and that is why they often come that way. (source)

As far as uses go, bakery emulsions keep the incorporated flavors more stable while your mixture changes temperature, and they combine more easily with other emulsions (butter, sugar, egg for example) than extracts do. As this book indicates those characteristics make them especially useful for pastry cremes.