emulsion is lighter texture of moisturizer(emulsion texture is like watery lotion or lotion but runnier),usually they are water based or water content,the function is pretty much the same (seal hydration on your skin),some people replaced moisturizer with emulsion,and some people also used moisturizer after emulsion,since the texture is lighter than moisturizer maybe emulsion wil suit oily skin more(dry skin can also used this),so it’s your choice which texture or consistency you like to be your moisturizer either emulsion or moisturizer