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I'm not too sure how I can expand on this.
Let's say that you want to tag the actual transaction with 'Rent' or 'Bill'.
Is there a field where I can insert a string for the other party? Note that this doesn't need to be hashed, it can be clear text.
This isn't strictly true, is it? For example, you could embed arbitrary data in a custom output script. But it would be more a kludge than an intended feature, and since the transaction would be nonstandard you'd have extra trouble getting it transmitted and confirmed. But people have included data in the block chain this way before. – Nate Eldredge – 2014-04-02T19:16:36.067
we have prunable outputs as of Bitcoin core 0.9 – Mark S. – 2014-04-02T19:29:40.117