Defaults and allowed fitting methods for the MARSS() function are specified in the file MARSSsettings.R. These are hidden thus to see them preface with MARSS:::.
allowed.methods is a vector with the allowed method arguments for the MARSS() function.  kem.methods and optim.methods are vectors of method arguments that fall in each of these two categories; used by MARSS().  alldefaults is a list that specifies the defaults for MARSS() arguments if they are not passed in.