Why does IPTC headline and 'Post Title' show same metadata after import

I am new to LR having moved from IDImager. After importing series of photos I find the IPTC Metadata Headline and 'Post Title' show same metadata after import. Is this because they contained the same data before the import or does LR sync the two fields? I am not familiar with Post Title field is it part of another XMP data set and if in LR I wish to update the Headline which should I use?

Thanks

Paul