Select the semi-dynamic ad structure ('Include Dynamic Prefix' check box) when your keywords require a prefix in either the ad headline or description line. Choose a general dynamic prefix that makes sense when attached to your keywords.
Example: Choose this type in two cases:
a) You are placing keywords for multiple products under the same ad-group. (Ex. your ad groups are defined per brand. Each brand may have more than one product, and therefore multiple product keywords)
b) Your keyword is unable to stand alone (without a prefix) Ex. “Marina” has multiple meanings, but if we add “Hotel” as a prefix then “Hotel Marina” better specifies the product being advertised.