Configuration Settings Question

[Freejoy]

Hi,

I love you extension, seems to work very well. 

I have a question concerning the last part of the configuration settings i.e. Strong Identifier Type, Use Existing Attribute and Attribute. I want to make sure I have these set correctly. I would like to use UPC -- I think that is what I need (12 digit number) I believe those are the ones i have access to mostly. The 13 digit numbers seem like they might be better but I live in US and what I read is that the UPC is for the US, but I sell outside of the US too. 

Anyway, I don't see an option for UPC. Maybe it's not important, don't know. I'm just trying to figure out the optimal settings. Right now I have the Strong Identifier Type set to EAN_UCC-13 but I only have the 12 digit UPC numbers. Will that still work?

On the next setting Use Existing Attribute I have set to No. I'm not totally clear about what this setting is for because the next setting Attribute has this drop-down list of what seems to be a listing of all my available product attributes. If I set Use Existing Attribute to NO does that mean not the use any of the attributes in the drop down list or not to use Strong Identifier Type? Because if I set it to Yes I get the following error in my system log:

ERR (3): Warning: strlen() expects parameter 1 to be string, array given  in /chroot/home/html/includes/src/Semantium_MSemanticBasic_Model_GoodRelations.php on line 865

...even though I have input a 12 digit UPC number for the product. Is the error because it is suppose to be a 13 digit number?

Last, if I set Use Existing Attribute to NO so I don't get the error does that mean I'm not using Strong Identifier Type and if so what is the best attribute I should select in the  Attribute drop-down box because it would seem I need to select them all. 

Thank you very much for your time and this extension.