Source: A TL125 website of which I unfortunately lost the URL

All parts on a Honda motorcycle have a unique part number. Honda divides parts into two broad
categories, General Parts and Standard Parts. General parts are those unique to Honda, such as valves,
pistons, gas tank etc. Standard parts are parts that follow universal standards, such as nuts, bolts and such.

General Parts

General parts have a part number divided into 3 sections, such as 22201-166 -000 (clutch friction plate for
the TL125).

• The first string of digits (2220 1-X:XX-XXX) indicates the Honda function and component number
  for this part. For example, there are several different part numbers for the TL clutch friction plate,
  all sharing funtion!component number 22201. Clutch steel plates have f/c code 22311, indicating
  same function but different component.
• The second string of digits (:XXXX.X-166 -XX.X) is the product code, also known as Parts
  Classification Number in hondaspeak. This is the number that indicates the product (motorcycle)
  where this particular part was first used. See below for a complete listing of older Honda product
  codes. 166 indicates that the TL clutch friction plates where first used in Honda MB5 motorcycle.
  Honda often lists several different part numbers for the same part, indicating slight differences in
  the part, such as different supplier etc. For example there are four different part numbers listed for
  the clutch friction plates.
  
  All TL125 models (1973 K, 1974 K1, 1975 K2 and 1976 S) share the product code 355. All
  General parts having a different product code are "recycled" from other Honda models. Only parts
  with code 355 were originally designed for the TL125, but may have been used in later Honda
  models. The product code is probably the most interesting portion of the part number when trying
  to determine interchangeability of Honda parts between various models.
• The third string of digits (XXXXX-XXX-000) is the Classification Number. I believe it is mostly
  used to indicate various suppliers or improvements for the same part. Also any potential color code
  is tacked at the end of the classification number as digits (letters) 4 & 5.

Note that some pretty universal parts are still listed under General Parts number. For example most (all?)
TL oil seals carry a general parts number. Luckily Honda parts fiche lists the actual dimensions for parts
like this.

Standard Parts

Standard Parts have a part number divided into 2 sections, such as 93500-06040 (6mm x 40mm pan
screw, for example TL125 clutch side engine cover).

• The first string of digits indicates the function/type of the standard part, for example all pan screws
  are 93500-XXXXX.
• The second digit indicates dimensions for the part.
• Standard parts occasionally have a third string of digits indicating the ISO standard designation
  and chemical surface treatment.

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Honda Motorcycle Product Codes

Following is a list of Honda motorcycle product codes. They are directly from an article posted on the
VJMC (Vintage Japanese Motorcycle Club) website, typed in by Ellis Holman. I don't update this list and
looks like it is no longer available from the VJMC website.

Addition: Richard Pade has found a more comprehensive list at CMS (Classic Motorcycle Supply).

Look for the model codes at http://www.cmsnl.com/faq.php?s_question=&id=16&fq_catid=14#16