when does a seedling become a plug plant?

as the title .

i know it may sound stupid to a expert or even a amature but i am a novice at this so i want to be able to refer to things as approriate as possible , cheers .

 

Hello noksucow
generally there are 3 sizes micro plug , standard plug and maxi plug . Different companies have other names and sizes . The site below shows some types. Plug plants will need potting up and growing on for a few weeks before they can be planted outside. Hope this helps.
http://www.brooksidenursery.co.uk/plug-plants-size-guide-4-w.asp

 

We amateurs grow seedlings and 'pot on'. Gardening businesses grow plug plants to catch those who can't be bothered to grow seedlings ! Cheers, Tony.

 

if you are dealing with Catalogue companies SOME (there are good ones) sell "Garden Ready " plugs. They are anything but.
In my experience all plugs need to be grown on BEFORE planting out.