Pete - this is conclusion I made based on Alamy Measures when photo gets returned in search for search term I had in title, but not in keywords.  Since keywords are obviously 2 level (10 super then rest) it is logical to assume title might have the heaviest weight for search engine.  It also fits overall Alamy model where phrases are not supported, in other words title string is parsed and indexed as array of individual words 

It is interesting to note how different agencies treat indexing;   IS does not index title/description at all, neither does AS.  DT on the other level puts by far the heaviest weight on Title, then Description.  I believe SS indexes title and supports phrases, but does not have "super" tags like AS (first 10) and Alamy.  IS on the other hand has this "managed dictionary" which is IMHO as wrong as it gets.  But IS will index tag that is not recognized, at least this is what their support told me once. So for instance if "landscape" is not recognized and they want you to pick "landscape - scenery" instead - don't bother.  Just "landscape" will get indexed fine.

Yes, many places don't use Title (the field) at all, just Description. For Adobe, they read Title and don't use Description, unless Title is Blank. SS reads Description (and calls it title) but ignores Title. I don't know if anyplace even looks at file name anymore? Kind of senseless since it could be ccws06-985 or nothing of use.

I don't know what IS sees or uses or if they know? The whole CV thing leaves blanks for words that aren't in their Vocabulary. I'd have to do a modern test and add a word, then see it it comes up in the search. Just like you, the best information is from personal testing.

So now I need to see what's up at Alamy this year. They changed things. I had to go through and alter over 2000 images, when they decided to stop searching Description, because people were writing a short story in there and all kinds of bad search results. Instead of saying, hey artists, stop spamming the field, they just stopped using it. Well thank you, now I'm screwed because I had names and places in the descriptions.

Next when they still had that terrible three box system, I'd put the major words in the first, like were were supposed to do. Then more general in the second and the third box, which was not displayed, was still searched. I used that for Names and locations, teams and specifics, which made the results less likely to have false hits. (If all that rank bs clicks, views, zooms was real?)

What happened? They changed and made the third box visible and made the second box invisible. No reason why? I wanted those box three details so be hidden?

Right, now the new system, no more boxes, Supertags and the bogus discoverability. Wonderful they just merged all the boxes = so now many of my images have spammed up keywords. Gee thanks again Alamy, now I can go through thousands of images, yet another time to make them right?

Maybe during the week, I'll take some files and play the tag/caption game? 

https://www.alamy.com/blog/captions-and-tags

Official position:

What are supertags? These are the tags that you feel are the most relevant to the image. In Alamy Image Manager, you can add up to 10 supertags and the search engine will place extra priority to these. Make sure you select the most relevant words only and use them wisely.

I�ve tagged my image �Banff National Park�, will my image show if someone searches �Park�? Our tagging system does not exclude constituent words of a tag from being searched. For example, �Banff National Park� will still show up for �banff�, �national park� and �park� searches. This also means that �Rhinoceros Beetle� will show up in �Rhinoceros� searches, but the search engine tries to make sure that the beetle image does not appear too high up.

What about plurals and stemming? The Alamy search engine doesn�t �stem� search words. This means that if a customer searches for �Cat�, the search engine does not extend this to �Cats�. If the plural is important to the image, then you should add it as an additional tag. Consider this carefully though � you could potentially be giving the customer irrelevant results which can harm your ranking in the future. If you have a lifestyle shot of a child playing on a swing, �Child� is a perfectly acceptable tag, but �Children� is not � a customer searching for �Children� will likely want to see just that: more than one child.

I saw searches by Alamy users for 'central park' where my photos showed up but that was for New York, mine was for central park in Boston UK so I had to change it to 'park' only.   

That's right. But they also don't stem, while others do.

So if @zeljkok  is correct, I'll need to pay much closer attention to the Description field on Alamy. (which they call Captions) And it's limited to 150 characters, on Alamy, by the way.

Oh Edit: DT uses all the fields. Title, Description and the Tags

Pete - bottom line is every agency is free to use whatever model they want.  Personally I find the simpler the better.  This is why ultra convoluted and unnecessary IS managed dictionary is the worst IMHO.  AS in other hand took minimalist approach, which is in line with overall UI philosophy of their products. Too minimalist perhaps

Forums like this are good though for us to exchange information and learn things. I am happy to share whatever I found playing with my port, regardless of fact that it might help direct 'competition'