In short : "hard" can be considered a confirmed booking, "soft" a tentative one.
Its more subtle that that, but thats the general idea.
Whether (functionally) you even need to think about hard/soft (and mixed) depends on YOUR organisations "resourcing model" - i.e. how YOUR organisation wants to add resources to projects - its all a functional discussion.
For example, you might let your Project Managers soft book resources to projects and only let you dedicated Resource Managers hard book them. (just an example).
Some more discussion here;
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