Default values are given by the XSD (and shown in the web UI) for many optional fields. There are 2 types of default value.

Static defaults (given by default="x") are an exact value that will be used if nothing is specified in the config.

Dynamic defaults (given by fb:default="something") contain an explanation of the behaviour that will occur if the value is omitted, but there isn't a single constant value that can represent it. For instance many of the error logger options have "As event" for their default, which means they will log to whatever the corresponding event logger is set to. This can't be a fixed value as we don't know which logger a user would choose at the point of writing the XSD, but this behaviour is generally useful and so the default is determined when the config is loaded.