Cannot say about tomatoes but it is so strange that when you go one the web and trawl 'rootstock'......no specific plant root is mentioned, very weird. Whitethorn? Hawthorn? what thorn? Maybe all sorts of root are trialed to check how much they limit or promote growth. So I understand your puzzlement.

Well m9 rootstock is used for apples but which tree it actually is no idea.

You say your tomatoes have 'gone bad' do you mean they got blight or another disease?

The M9 rootstock used for Apples is an Apple. These root stocks were bred at East Malling They were selected from 1,000's of seed grown Apple trees because they had certain genetic patterns of growth, height, vigorousness and fruiting etc.
Other trees have differently named rootstocks, Pixie for example for Plums. There are ones for Cherries, Pears, even Medlars.
Roses have rootstocks, usually Briar roses.
Having said that generally the rootstock is from the same genus as the top growth, though not always, but even then they must be close enough for them to bond.

<p> thankyou for your help. the tomatoes wasn,t blight they developed very well but the tomatoe,s themselves went black and started to rot. eamonn

If you are sure that the tomatoes didn't suffer from blight, which was very bad for outdoor plants, then I would suggest that the problem could be 'blossom end rot' which would make the fruit go bad. Calcium deficiency would be the culprit which can be cured with a dose of garden lime.