Libraries that are in RunTime format are protected in that block source code is stripped from each block making your custom source code inaccessible to the end user.
Model Developer Editions (MDE) of ExtendSim cannot fully use libraries that have been converted to RunTime format. (MDE users can open a model that uses RunTime formatted libraries, but the end user will only be able to run the model, not make changes.)
To convert your custom libraries to RunTime format (Note: RunTime versions of ExtendSim ship with RunTime versions of all ExtendSim libraries so you will only need to convert any custom libraries you might build.):
In ExtendSim, choose Library > Library Tools > Convert Library to RunTime Format.
Select the library for conversion. ExtendSim will make a copy of the library, convert it to RunTime format, and save it in the same location as the original library.
Libraries converted to RunTime format keep the same name as the original, but add the extension .lbrrt.
Since the library name is the same, models opened by the Analysis RunTime, Player RunTime, Student, and Model Developer Editions of ExtendSim will recognize the reformatted libraries as if they were the originals. (Note, however, that MDE versions of ExtendSim will not allow the end user to build models with RunTime formatted libraries.)