Engagement Party or Wedding Shower
A party is a party, but each party is different from all others. It is up to the host—perhaps with the engaged couple’s assistance—to design the party as they like. And it is up to the host to convey to the guests what to expect at the party. After all, nobody likes showing up in formal attire when everyone else is wearing grass skirts and preparing for a luau.
Specifically, the host needs to state whether the upcoming celebration will be an engagement party or a wedding shower. At a shower, the couple (or one member of the couple, as in a bridal shower) is “showered with gifts.” Usually a shower has a theme, and guests are encouraged to find engagement gifts to fit the theme. Sometimes the presents are even opened at the party, though this is frowned upon for several reasons.
An engagement party can also have a theme, but guests are not expected to bring gifts, and any gifts would absolutely not be opened during the party.