I write about this because even experienced volunteers (7 years servitude!) are getting things wrong. Unfortunately, some of it is subjective and in some areas I do not agree with the Free Census policy. It is all covered in the General Instructions that everyone, including checkers, should have.
This can be divided into two - 1841 - and the rest.
In 1841, there are two fields. The first one asks if the person was born in the county or not. If the answer is YES, then the enumerator should enter a "Y". We enter the county code. If the person being recorded was born in England or Wales but not in the county, then the enumerator should record N for NO. We just enter OUC or the county mentioned. The enumerators were not required to enter the county. However, if the person was born overseas, or in Scotland or Ireland, then the enumerator should use the second column and enter F, S or I. We use OVF, SCT or IRL. If there is additional information, then record it in the notes column. If there is no information or you can't read it, enter UNK.
After 1841, things changed. There are still two columns to fill. The first should be the county and for this you use the Chapman code. Free Census has a modified version of this table; modified to include OVB, OVF and UNK. The second column should record the town or parish. For Ireland, Scotland and Wales, the enumerators were only required to enter the county, not the town or parish. For some reason unknown to me, ALL volunteers are strangely reluctant to use the Chapman code for Irish and Scottish counties. In fact, hardly anyone does, so I have to input them. If the second field is not used by the enumerator, input a hyphen.
Overseas births are difficult. This is a Free Census thingy and is not "as is". In general, I subscribe to the idea that if the name looks English, then they are OVB no matter where they are born. You, of course, still enter the details of their birthplace in the second column. In the first, you enter OVB or OVF. Quite often, British Subject or B S are added to the place of birth. You should enter this information, even if you are entering OVB. You should also enter OVB for people who are described as naturalised. If they are born at sea, then it is OK to follow the rule of thumb and enter OVB or OVF; but you can also enter UNK.
Free Census supplies a gazetteer, but it only has 35,000 or so place names. It is possible for the validator to enter the modern place name, but I don't. As long as you are sure it is transcribed correctly, then it will pass through the system and appear online, just as the enumerator wrote it. It is up to the customer to work out where the place is. If you are having trouble reading the place name, then there are various aids you can use. The first is Google. Just type the place name in and see what happens. Quite often, Google will put up our census returns in the first ten hits. So, another way of checking on a place name is to search our online returns. English place names have been largely unchanged for a long time. Sometimes, it is worth just browsing our online returns for clues. Some volunteers use the LDS familysearch web site; others use FreeBMD. If you think it will help - then leave a note. Do not correct the enumerator's words!
If the place of birth fields are empty, then you enter UNK hyphen. If it says N K or Not Known, then you enter the respective words and UNK for the county. In fact, whatever it says in the place of birth field, that is what you should enter. I will also point out that it is very unlikely that the enumerator would enter U K, as United Kingdom was not used then as it is today. It has got to be a badly written N K. Sometimes you will see N B; this is for North Britain, usually meaning Scotland. If you can't get everything into the place of birth field, then split it and use the notes column. Try and make it so that it makes sense. If you have something like St Stephens by Launceston, then St Stephens by Launc is acceptable. You can also spell out the whole thing in the notes column. Please don't use St. when the enumerator has used St - it makes hard work for me.
The bottom line is that we want what the enumerator has written; not what we think he should have written. As is!!!!!!