I don't know if I understood correctly, in Marcus's source I didn't identify particular paths, in any case even if I run the source from different paths, the compilation happens successfully !
Let's try an clear up a few things
Using the name Marcus (at least to me) works confusing. ALB42's real name is Marcus and MarcusF's real name is also Marcus.
They are two different individual persons and should therefor not be mixed (that is why I explicitly try to use the name ALB42 instead of Marcus when referring to Marcus S instead of Marcus F).
The "path" I was talking about is set in the FP-IDE as shown in the screenshot you attached when I asked for it (for which I thank you very much).
So, it has nothing to do from which directory you invoke/start the fpc or fp executable but which directory fp and fpc uses to locate their (3th party installed) units. As that is defined by a set of rules for which you need to read the official Free Pascal manuals in order to understand how that is suppose to be set up correctly.
ALB42 apparently already did some pre-work for us in that regards and, as such we are more or less obligated to use it (unless someone choose to do it differently and use a custom setup as is allowed to be done by anyone as long as the set rules are followed).
MarcusF (thus not ALB42) already asked (in another thread) a similar question about MUIClass the day before you did and he understood the path to MUIClass needed to be added/adjusted for his personal FP-IDE setup, he just not understood why initially it did not work for him and so asked for help. He eventually managed to solve the issue and he knew how and where to do that.
Now that he knows, he is able to do the same for any other (3th-party) package as the process is similar and will always work while the solution you choose to use might seem to work initially but, on the long run is not maintainable and for sure will cause errors as that general bin directory is used to store compiler binary executables used by the compiler while any unit name stored in there with the same name has the chance to overwrite any of the executables that are stored in there simply by means of compilation (and that will cause errors for end-users and such errors that are very difficult to detect/recognize because no one in his/her right mind would store units in bin(ary) directory that you now have used).
And then I did not even mentioned the fact that units are suppose to be stored on a per platform base so compiled units should always reside in a subdirectory of the platform name (i386-aros, x86_64-aros, arm-aros etc) and which logic your chosen solution defies.
As you all know I am not a developer, in my opinion the Binary "FPC" is faster, in Def_PAS, but "FP" allows you to do other operations on the source q
Yeah it is a shame that it is not possible (by default workbench) to choose which action to perform for a known file-type (I know that magellan desktop replacement can do it). It is all in the eye of the beholder what would be the best cause of action so your solution to use fp as a tool to open the .pas file is as good as any (no complaints there

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