This tag should be used for questions that are relevant to most or all blockchain based cryptocurrencies. If your question is specific to one altcoin, please use/create the tag for that altcoin.
Alternative cryptocurrencies are cryptocurrencies that are not Bitcoin. Altcoins typically use a blockchain similar to that of Bitcoin's. Many use a similar Proof of Work scheme but with different algorithms. Others use Proof of Stake or some hybrid between PoW and PoS.
This tag should be used for questions that pertain to many cryptocurrencies, including Bitcoin. These can be generic questions about how things like PoW or PoS work. Questions about a specific altcoin should not be under this tag.