A feature curve in its most basic form is a two dimensional cell which has two vertices at either end. They are exclusively used in surface meshes to indicate where sharp edges exist and/or to define in-plane surface details that need to be maintained in the final volume mesh.
When imported or created in STAR-CCM+, feature curves are listed under the Feature Curves node for each region. An example showing two sets of feature curves is shown below:
If the surface remesher is used to improve the starting surface quality then including feature curves is critical if sharp edges and/or surface detail (such as imprints) are to be preserved. If the feature curves are not included then edges are rounded off and surface detail will be lost.
Including features curves for edges when utilizing the surface wrapper is not as critical as the wrapper feature angle, which can be used to globally extract and maintain sharp angles. However, for features that have an angle less than that specified or for surface details such as imprints, then it is recommended that feature curves be used to preserve the definition.
You can have as many feature curve entries as you want for the surface. However, it is typical that only the critical edges (say those over 45 degrees) are defined by feature curves so as to limit the complexity of the model.