Pending State
When a transaction is submitted to the Ethereum network, it first goes into the pending status, waiting to be executed by the nodes. A transaction can be in the pending state for a longer duration if the gas price is set very low in the transaction and the nodes are busy processing other higher gas price transactions.
During the pending state, the transaction initiator is allowed to change the transaction fields at any time. They can do so by sending another transaction with the same nonce.
Prerequisite Readings
DEP vs Ethereum
In Ethereum, pending blocks are generated as they are queued for production by miners. These pending blocks include pending transactions that are picked out by miners, based on the highest reward paid in gas. This mechanism exists as block finality is not possible on the Ethereum network. Blocks are committed with probabilistic finality, which means that transactions and blocks become less likely to become reverted as more time (and blocks) passes.
DEP is designed quite differently on this front as there is no concept of a "pending state". DEP uses Tendermint Core BFT consensus which provides instant finality for transaction. For this reason, Ethermint does not require a pending state mechanism, as all (if not most) of the transactions will be committed to the next block (avg. block time on Cosmos chains is ~8s). However, this causes a few hiccups in terms of the Ethereum Web3-compatible queries that can be made to pending state.
Another significant difference with Ethereum, is that blocks are produced by validators or block producers, who include transactions from their local mempool into blocks in a first-in-first-out (FIFO) fashion. Transactions on DEP cannot be ordered or cherry picked out from the Tendermint node mempool.
Pending State Queries
DEP will make queries which will account for any unconfirmed transactions present in a node's transaction mempool. A pending state query made will be subjective and the query will be made on the target node's mempool. Thus, the pending state will not be the same for the same query to two different nodes.