Special stamp in my passport.. what does it mean?

Hi all. Upon entering the UK a few weeks ago with intent to stay for 5 months, the IO granted me entry but stamped my passport with my landing card number, which I presume has everything we spoke about. (Hopefully not everything as I was so nervous and fumbled my words) the line underneath says "six months".

Just curious as to if this will affect a future trip to Paris in these next few months while I'm here and leave/re-enter the UK. Or will it affect a future fiance or work visa?

Thanks in advanced! Just want a little more understanding on this stamp.

ALSO.. is this 5 months out of the 6 allowed in a calender year or 5 months beginning from the day I entered the UK? If so.. we plan on marrying here so would a Marriage visa bypass that?

If you leave to go to Paris, you'll invalidate that stamp and have to present yourself for entry clearance at the border again when attempting to come back to the UK and yes, a special stamp can affect your ability to be readmitted to the country.

It means you're allowed to stay 6 months from the day you entered the UK, but that they had questions when you entered, and next time you enter they can refer back to your landing card to see if you stayed consistent with what you said. If you go to Paris, this entry stamp (which is actually a visa, you just didn't have to apply in advance) technically ends, and when you re-enter, you will actually be asking for a new entry clearance. At that point you can expect more questions, and they will probably look at the landing card referenced on our stamp. It won't affect a future fiancé or work visa. These are single entry visas, so once you left the UK, any future visas you applied for would supersede this one.

If you apply for a fiance visa you will be allowed back in even if you have been in the country for your allotted 6 months, I'm not sure about a MVV though.

The 6 out of 12 months is a rule of thumb, and does not go by the calendar year.

Just curious as to if this will affect a future trip to Paris in these next few months while I'm here and leave/re-enter the UK. Or will it affect a future fiance or work visa?

I can attest to the fact that those 'special stamps' don't affect future visas.  I had one of them whilst returning to the UK with my then-BF and I had my Fiance Visa approved 6 months later. 

ALSO.. is this 5 months out of the 6 allowed in a calender year or 5 months beginning from the day I entered the UK? If so.. we plan on marrying here so would a Marriage visa bypass that?

If you plan on marrying in the UK whilst on a visitor visa, you'll need to be honest with any IO who asks you about what your plans are. 

You should have a MVV or Fiance Visa to get married in the UK.  It's only really allowed if you come to the UK without plans to get married and you make the decision once you're here. 

So, be honest with the IO what your plans are- if you plan on marrying in the UK on a visitor visa, you might have a more difficult time at the border (and may not be admitted) but if you lie about your intentions, you could be in even more trouble (banned for 10 years for using deception).  Honesty is always the right thing at borders and if you need a visa, you should apply for it.