Important/interesting IPTC tags/placeholders, according to IPTC specification:
Placeholder Tag name/function
$I5 - Object Name
$I7 - Edit Status
$I10 - Priority
$I15 - Category
$I20 - Supplemental Category
$I25 - Keywords
$I30 - Release Date
$I35 - Release Time
$I40 - Special Instructions
$I45 - Reference Service
$I47 - Reference Date
$I50 - Reference Number
$I55 - Created Date
$I60 - Created Time
$I65 - Originating Program
$I70 - Program Version
$I75 - Object Cycle
$I80 - Byline (Author)
$I85 - Byline Title
$I90 - City
$I92 - Sublocation
$I95 - Province State
$I100 - Country Code
$I101 - Country
$I103 - Original Transmission Reference
$I105 - Headline
$I110 - Credit
$I115 - Source
$I116 - Copyright
$I120 - Caption
$I121 - Local Caption
$I122 - Caption Writer