Salford appears before Manchester

This question keeps coming up in related guises. It's merely a consequence of the underlying algorithm for label placement, and nothing to do with the data (the same phenomenon also can be seen on Google maps). I suspect that shorter names are more easily placed without conflicts which bumps nearby longer names away from the renderer. This usually only applies at a small number of zoom levels.

Similar questions:

• how-country-name-is-selected-and-displayed-at-low-zoom-levels-for-small-countries-for-instance-cyprus
• philadelphia-does-not-show-up-but-camden-does

There are many more but tend to be hard to find because the question is usually asked about relative rendering of two specific places.

There are two bits to this I suspect - one is how Salford appears on the "standard" layer of the OSM website, and the other is how Salford and Manchester should be tagged.

Taking the second part first, essentially the usual (though not universal) OSM definition of a "place=city" is a place that's substantially larger and/or more important than nearby towns (though both criteria can be subjective and open to dispute).

There were discussions on the talk-gb list about towns and cities back in April, prompted by someone attempting to "tidy up" European town/city tagging with no local knowledge. Some of the changes made sense; some did not. Since that discussion I had been intending to try and produce a list of "town/city discrepancies" but haven't had the chance unfortunately, so since then city/town retagging has essentially continued piecemal (as with Salford).

Moving on to the renderer, there are a number of pieces of information that a renderer could theoretically use:

1. What it's tagged as ("city" or "town").
2. What the area of the way or relation that matches the city name is (the Mapnik software that the "standard" layer uses can process at least "way area" when making rendering decisions).
3. Other tags, such as population, although the tags that the "standard" layer has access to are limited.

Issues relating to the "standard" stylesheet are here. I'd be very surprised if there weren't already discussions there about town and city tagging.

On the more general "how to tag" question, what I'd do now with regard to towns and cities is to try and come up with a local concensus about tagging, especially in areas that aren't local to you. There may be very good reasons why, for example, Telford is tagged as a town rather than a city. I'd also try and make sure that all of the information that renderers in the future might want to use (population numbers, wikipedia and other links) are included if there's a suitable source. It might not make a difference to the "standard" layer tomorrow, but the more data captured the more chance future renderers have of making the right decision.