These are the extended parameters of a feed. None of them is mandatory; you might just skip them.
Some are default parameters for new messages.

Field name Purpose Constraints Example(s)
Link A link a user may click when the feed header is displayed by a feed reader. E.g. a link to a page describing the feed in more details, a feed to the home page, or a link to the feed itself. Optional.

Max. length: 255 characters.

Must be a valid URL.

http://www.yourdomain.tld.en

http://www.mydomain.tld/de/feeds

Copyright Copyright of the feed. Optional

Max. length: 255 charac­ters.
Variable references are allowed.

Copyright (C) MyCompany

Copyright {copy} {year}, {company}

Publication time Publication date and time of the feed. Automatically updated when new feeds are added. Op­tio­nal.

Format: "yyyy-mm-dd hh:MM:ss".

Default: now.

2010-11-01
Time-to-live The time in seconds a feed reader may cache this feed (incl. messages). After this time it should re-read it. Optional.

Numeric.

Max. length: 5 digits.

Default: 60

60
Maximum age Default difference in seconds between the start and the end publishing date / time of a message. Optional.

Numeric.

Max. length: 10 digits

Default: 604800 seconds = 7 days.

604800