What were 'high-priority' transactions in Bitcoin Core v0.9? I came across these release notes for Bitcoin Core 0.9 and saw this quote under 'Transaction Fees'. Does this distinction between 'high-priority' and 'highest-fee' transactions still exist? What exactly were 'high-priority' transactions?


  Note that getting a transaction relayed across the network does NOT guarantee
  that the transaction will be accepted by a miner; by default, miners fill
  their blocks with 50 kilobytes of high-priority transactions, and then with
  700 kilobytes of the highest-fee-per-kilobyte transactions.
Historically it was not required to include a fee for every transaction. A large portion of miners would mine transactions with no fee given that they had enough "priority". Today, low priority is mostly used as an indicator for spam transactions and almost all miners expect every transaction to include a fee. Today miners choose which transactions to mine only based on fee-rate.

Transaction priority was calculated as a value-weighted sum of input age, divided by transaction size in bytes:

priority = sum(input_value_in_base_units * input_age)/size_in_bytes


EDIT:

input_value_in_base_units = satoshis to be transferred (e.g 1000 sat).
One Bitcoin is worth 100,000,000 satoshis.


input_age = is how many blocks the input has been present for. An unconfirmed transaction 
has an age of 0, and one that has 100 confirmations has an age of 100. An input that
was confirmed 50 block before have input_age of 50



size_in_bytes = size of transaction