Sorry about confusion.

First of all, 'package' is perhaps the wrong name of entity. A couple of years ago I was suggesting renaming to 'unit' or 'module', but the name is already too stuck...

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1) Is a package a single build of an application/library etc. ?

I guess shortest description is 'minimal library module in source form'.

E.g. good example for starters is 'plugin/gif' - that is package that contains code dealing with 'gif' format.

Of course, some packages contain more stuff, like 'Core', which contains all U++ basics.

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2) Why is something a main package and something else is not a main package ? Is it because a main package can have dependencies but is never a dependent of another package ?

Main package represents (in most cases) application. It is 'main', because it is the one package you choose when starting theide. Rest of packages are put into the project based on dependencies.

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3) For any given package how can I see its hierarchy of dependencies ?

In 'Package organizer'.

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4) Given a package already created somewhere in a directory, how do I add it to the IDE so it knows about it ?

Package is a directory with some source files and ".upp" file, which contains description of package (list of files, dependecies ('uses'), less often optionally external libraries, compiler options etc...).

Nest is directory which contains (at first level) packages.

Assembly is an ordered set of nests (so that all packages of all nests are combined into single 'namespace').

Now I am not sure how your question is related to these concepts, but let us say you have a directory of source files and you want to 'convert' that to package.

I guess simple way to do that is to create a fresh empty package, copy source files to package folder, then add them into package using right click on list of package files and choosing 'Add package directory files...'.