What is the XCM format on Polkadot? Could anyone please explain? Thank you so much!
XCM stands for cross-consensus message, and is a message format designed to let multiple different consensus engines communicate with one another.
XCM is not specific to Polkadot, however is particularly useful for Polkadot in combination with XCMP, which is the cross-chain messaging protocol. But note that the "C" in the acronym has changed, and that is to emphasize that XCM really could be used for scenarios beyond just cross chain communication, for example having two smart contracts talk to one another.
The best way to learn about XCM is by reading the specification: https://github.com/paritytech/xcm-format
As a high level simplification, you can think of XCM as standardizing all the most common calls you might expect between two consensus engines. For example, transferring balances from one another (which itself may be composed of multiple steps like querying a balance, withdrawing a balance, depositing a balance, etc...).
Imagine you have two different blockchains trying to communicate, and each of them are constantly upgrading, adding new features, changing their runtime, etc... all of course possible because Substrate makes it so easy to upgrade a blockchain. In the situation a unified language like XCM did not exist, these two chains would struggle to keep up with one another and establish a consistent communication method. One chain could change the Pallet they use for managing balances, and without coordination, this could lead to lots of errors and funds lost.
With XCM, chains can simply wrap all of their chain specific logic around a standard message format, and ensure that intentionality is not lost over the wire.
The XCM format is designed to be upgradable and (mostly) backwards compatible, and uses a versioning system to note changes to the format.
To add onto Shawn's detailed answer, the XCM format is comprised of a set of instructions that convey semantics and optionally contain operands that is defined by the spec for the chain that processes the instructions.
For example, one of the basic XCM instructions is
WithdrawAsset, and if we take a look at the spec, it says the following:
Remove the on-chain asset(s) (assets) and accrue them into Holding. Operands: * assets: MultiAssets: The asset(s) to be removed; must be owned by Origin. Kind: Instruction. Errors: Fallible.
The first paragraph in the spec denotes what the
WithdrawAsset instruction is aimed to do -- remove on-chain assets owned by the origin (i.e. the account that submitted the XCM instruction) and put it into the holding [register].
The next item there lists out the operands that
WithdrawAsset is expected to contain. We see that there's only one operand called
assets, and it is used to indicate the assets -- both the class and amount of assets -- that we want to remove from the origin.
After that, we see the kind of XCM instruction that
WithdrawAsset belongs to. Despite it being called "XCM instructions", some of them are not strictly "instructions" in the sense that they are used to orchestrate actions or behavioural changes;
ReserveAssetDeposited would be one such XCM "instruction" that is not a kind of instruction, but a kind of "trusted indication", as it is used to truthfully inform its destination that some assets have been put into its account on the reserve chain. Needless to say though,
WithdrawAsset is a kind of instruction.
Finally, we are informed about whether or not the instruction is fallible or not. Later versions of the spec may eventually add the kinds of errors that we would expect to see when errors are thrown whilst processing an XCM instruction.
What you won't see in the spec here is how exactly the consensus system that executes the XCM instructions is supposed to process them; that is left for the implementors of the consensus system to decide. To give a concrete example with
WithdrawAsset, the spec only defines the format of the instruction and the purpose of it. The exact functions to call or modules to include is not mentioned at all within the spec, meaning that it really doesn't matter if the implementor has used the balances pallet, the assets pallet or even some other mechanism to withdraw assets from the origin account, as long as the methods used fulfills the objective of "removing the on-chain assets and accrue them into the holding register".
A corollary to what I have mentioned above is that implementors may even introduce side effects while processing a particular XCM instruction, such as making logs of the instructions that it has processed, or firing off events based on conditions, etc. In XCM, we simply don't quite care about these side effects, as long as the consensus system can produce the specified semantics.