This feature allows you to give your clients a choice of making their listing have no duration within the system. If this duration is chosen the listing will only close by action other than duration. Depending on the type of listing those "other actions" could be:

• classifieds - will only close if the client placing them or the admin manually close them through their requisite admin tools.
• standard auctions (buy now option or not) - don't allow. This feature is useless on standard auctions. Standard auction need a time to end bidding. The listing expiration is it. So if you allow standard auctions and still wish to use the unlimited duration for the classifieds option on your site you should "fall back" to setting durations within the "charge by duration" feature per price plan. That would allow you to set separate durations for classifieds and auctions as each has their own "price plan type".
• buy now only auction - This basically makes the buy now only auction an "until sold auction". Clicking the "buy now" will close this type of auction.
• inventory item auction - As with buy now only auction acts as an "until sold auction". But with inventory item auctions individual items of the full count on sale are sold to individual buyers. In this case the inventory item auction will only close when the last item is purchased.

You have the option to set unlimited duration in all current places duration is set.