When using the IGI online you need to look below the entry, at "Messages".
If it reads "Messages:
Form submitted by a member of the LDS Church", you will know that this has been put on by an individual, and may or may not be accurate [it may be a guess or a complete invention to fill in a gap in the family tree].

If it reads "Messages:
Extracted birth or christening record for the locality listed in the record", it is taken from the parish register, and the LDS film number is written under "Source Call No." which you can use to order the par reg film.

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