Breaking down a raw transaction:

Question

Complete example:

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Breakdown:

Version (4-byte):

01000000

---

Number of inputs (variable size but in this case it is 1-byte):

01

---

Previous transaction ID byte swapped (32-byte):

186f9f998a5aa6f048e51dd8419a14d8a0f1a8a2836dd734d2804fe65fa35779

---

prev_out_index (4-byte):

00000000

---

scriptSig length (variable size but in this case it is 2-byte which represents 139-bytes for the following scrtiptSig):

8b

---

scriptSig (variable size but in this case it’s 139-byte):

483045022100884d142d86652a3f47ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e381301410484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade8416ab9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc17b4a10fa336a8d752adf

---

Sequence number: (4-byte):

ffffffff

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Don’t know what this is:

02

---

Value to send (8-byte byte-swapped):

60e3160000000000

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Output 1 – Receiving address for amount above:

19 76 a9 14ab68025513c3dbd2f7b92a94e0581f5d50f654e7 88 ac

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Value for change (8-byte byte-swapped)

d0ef800000000000

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Output 2- Change address for amount above:

19 76 a9 147f9b1a7fb68d60c536c2fd8aeaa53a8f3cc025a8 88 ac

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LockTime (4-byte):

00000000

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Question:

The bit where I wrote Don’t know what this is, which has the value 02. I don't know what the purpose of that value is. What's the purpose of that value?

Answer 1

The bolded byte in the following raw transaction is the number of outputs (two in this transaction):

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

The Bitcoin.org Developer Documentation has an extended description of the raw transaction format if you'd like more information.

Answer 2

Don’t know what this is:

02

It's the number of outputs in the transaction.