“AltSl” indicates those languages which are similar enough, both in meaning and in spelling (e.g. Danish compared to Norwegian Bokmål), to be used as “alternate source languages”. That is to say, you could try looking up a word in their dictionaries and have a reasonable chance of success.
“AltTl” indicates those languages which are similar enough in word meanings (though not necessarily in spelling) to be used as “alternate target languages”. They could be understood and provide meaning or inspiration to a speaker of the language in question. For example, a speaker of Serbian who was trying to read the Italian Wikipedia might find an Italian to Croatian dictionary useful. Or a Scottish Gaelic speaker trying to think of a suitable Gaelic word might find an English to Manx dictionary useful as inspiration.