Different Types

Accruals Accounting

Mark to Market Accounting

Also called Fair Value accounting.
Any movement results in an unrealised profit or loss on that position.

A mark to market model is used. This refers to any valuation that is benchmarked, extrapolated, interpolated or otherwise calculated from market inputs
The purpose of "marking to market" is to determine the unrealised profit and loss by comparing the current market value against the following:
1) the previous valuation date (previous trading day)
2) the date of the transaction

This re-values a position held based on the current market price
The "mid market" price or average of bid-offer spread is often used
The "bid price" can be used in the case of a positive or long position as it's the price the position can be sold at
The "offer price" can be used in the case of a negative or short position as it's the price the position can be bought at

Unrealised Gain/Loss - After you buy an asset the price will inevitably change. If the price goes up you have an unrealised gain. If the price goes down you have an unrealised loss. It is only when you actually sell the asset do you have a realised gain/loss. Unrealised gain/loss may or may not ever be realised
If you buy a bond the accrued income on its coupon is unrealised. If you sell a bond then the accrued income from its coupon is then realised.

Hedge Accounting

This is making sure that all the transactions within a hedge must be accounted for using the same accounting method.

Invoice Discounting

where the factor provides finance against invoices as before but the client runs the sales ledger and collects the debt