As previously mentioned, there are three metadata groups—configuration, class, and recipe.
The first parsed metadata in BitBake is configuration metadata, identified by the .conf file extension. This metadata is global, and therefore affects all recipes and tasks that are executed.
BitBake first searches the current working directory for the build/conf/bblayers.conf configuration file, which is expected to contain a BBLAYERS variable that is a space-delimited list of layer directories. For each directory in this list, a conf/layer.conf file is searched for and parsed to add the recipes, classes, and configurations contained within that particular layer. LAYERDIR is set to the directory where the layer was found during this process and also adds this layer to the BBPATH list, which points to the set of directories with recipes, classes, and configurations.