All pools in Balancer are also ERC20 tokens known as BPTs (Balancer Pool Tokens), which represent proportional ownership in the pool's liquidity. When users add liquidity through
joinswap* they receive BPTs proportional to the amount of assets they are adding to the pool.
BPTs are an opinionated ERC20 token implementation, and have a few subtle differences.
transferFrom from itself does not require a previous allowance.