The cluster of buds bottom left are the flowers and should produce fruit. The long stem is a holding a leaf bud and the stems will grow on from there. I have heard them called water shoots before. new flowering 'spurs' will form on these shoots for fruit in a couple of years.

the rhs link is excellent.

https://www.rhs.org.uk/Advice/profile?pid=90

The photo may not show the bud on the end of the shoot clearly enough, but from what I can see, it is definitely a flowering bud because it is plump and downy. I have looked at the rhs website, as you advised, and it was helpful. There was also some very useful information on 'www.orchardorigins.org'. Thank you for your reply.

They don't usually flower at the tips of the branches though so prepare to be disappointed by that one! You're more likely to get flowers at the bottom end of that shoot.

Yes I see what you mean, because the tree is not a tip-bearer. So do you think that the bud will stop at the point where it is, while the shoot grows longer? I suppose that one way to find out is to pay close attention to what the tree does throughout the year, but this takes time and patience.
The following is taken from a section called Identifying Fruit Buds And Growth Buds on orchardorigins.org. which I found very informative.
Axillary buds
During the first year of growth, a new shoot produces embryonic buds, known as axillary buds, along the length of the shoot. These buds form at the base of a leaf – the leaf axil – hence, axillary buds. It’s much easier to see them once the leaves fall in the autumn.
Over the winter, axillary buds lie dormant. It is only in spring when growth hormones in the tree determine which buds will develop into a growth bud, fruit bud or remain dormant.

I agree, its difficult to get advice and to find good photos to help with pruning apples. Good luck!

I think you have suggested your own answer. If you have plants the best way of learning about them is to watch what they do. If you haven't time to do that you take the risk of things going wrong and leaving it too late to do much about it.
What you can expect to happen with your tip bud is for it to open and unfold at least two leaves, with another growth tip between them, which will gradually lengthen the original stem. This will need pruning next winter to encourage it to make flowering buds. Perhaps be a good idea to go back to your Orchardorigins site then for advice and diagrams on how to do it. Good luck.

If you plant a fruit tree, then go away and expect to come back to pick apples you are likely to miss other problems along the way. Gardening does indeed take time and patience.

If this is a newly planted tree in its first season it should not be allowed to keep more than one or at most two apples - It will need to use all the nourishment to establish a good root system before being allowed to bear much fruit. This will pay off in the longer term.

Thanks Steragram, I feel like the mystery of the 'terminal bud' that has eluded me for years has been unravelled at last! The one in the photo is not in fact a flowering bud, as I had thought. I could not understand how the shoot could grow longer, if this bud turned into apples. I can see what you mean when you say that the best way to learn about them is to watch what they do. You were right too Seaburngirl, it is indeed a leaf bud.
There is some good guidance called Fruiting Spurs on Apple Trees, with photos, by a gardener called David Marks on a website called GardenFocused, Cottagekaren.
My Jupiter apple tree is in fact mature and was planted about seventeen years ago.