My understanding is that if I'm neither the sender or receiver then you can pay an out of band TX to a mining pool which can be called boost or accelerate.

I can already do RBF from my wallet. What exactly is the boost/accelerate tx services doing different?

JuanMiguel

If you broadcast a transaction with too low a fee, another transaction with a higher fee is called a boost or accelerate. 

If you are sending, you can do replace by fee (RBF)

If you are receiving, you can do child pays for parent (CPFP)