Default parameters are a way to not have to bother with parameter names in parser functions. So instead of having a key value pair like
name=value, you simply have the value like
value. The big difference with regular, named parameters is that they need to be provided in a fixed order, which is determined by the parser function you use.
You can used default parameters mixed with named ones, but you need to be very careful not the get the ordering wrong with this approach.
For examples of default parameter usage, see the parser function documentation.