Updated: 15/Jul/2011; You prefer to read this in a KB article? http://support.microsoft.com/kb/2578058
Imagine the following scenario:
SharePoint 2010 with MySites and Managed Metadata Service configured. You created an additional User Profile property within your UPA settings. That custom property you want to connect to a TermSet which is stored in the Metadata Service storage.
At the end you also want to have the Term Picker button next to the custom field when a user will edit the own profile. Using the picker button is a nice feature to check all the offered values.
This will need to have the TermStore (Managed Metadata), where you are storing the TermSet and all the terms, configured as the default storage for Keywords.
If you have a different TermStore configured for the default storage of Keywords you will not see the picker button and it is currently an unsupported configuration.
The reasons to show the picker button: - We need to know the TermSet ID; that is possible. - We need to know the TermStore ID (the managed metadata service application) and that cannot be determined in the given way when editing the own profile. But we know the DefaultKeywordsTermStore and therefore we have to use that storage.
Please have this limitation in your mind when you plan a larger farm and also when you want to split the MySite hosts around the world.
Example: - Main farm is located in USA http://US-MySites - Second farm is located in EMEA http://EMEA-MySites - Third farm is located in APAC http://APAC-MySites
If you want to use for all MySites a CustomProperty and TermSet you have to configure the TermStore located in USA as the default storage for Keywords in all other farms. You can use a connection to a remote service application to configure those needs.
References: Plan to share terminology and content types (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ee519603.aspx
Managed metadata service application overview (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ee424403.aspx
Create, update, or delete a managed metadata service connection (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ee530391.aspx
Multilingual term sets (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ff678224.aspx