Hi Gareth,
Assignment of Asset Class to Company Code is not direct. First you assign Chart of Depreciation to a Company Code and then assign Asset Class to Chart of Depreciation. Below are the steps.
1) Create Chart of Depreciation: Path - SPRO --> Financial Accounting (New) --> Asset Accounting --> Organizational Structures --> Copy Reference Chart of Depreciation
2) Assign Chart of Depreciation to Company Code: T-code OAOB; Path - SPRO --> Financial Accounting (New) --> Asset Accounting --> Organizational Structures -- Assign Chart of Depreciation to Company Code
3) Define Asset Class: T-code OAOA. Assign Account Determination, Screen Layout and Number Range to Asset Class during creation of Asset Class.
Pre-requisite: Account Determination, Screen Layout and Number Range should exists
4) For specific chart of depreciation assign Screen Layout and Account Determination for an Asset Class using t-code ANK1. Highly recommended to create different account determination for each chart of depreciation in case you more than one charts of depreciation.
5) You can also deactive an asset class for specific chart of depreciation using t-code AM05
Hope this helps you
Regards
Pankaj Pareek