AbleCommerce Shopping Cart Software

I'm not 100% confident on these answers, but I am confident enough to post a reply

1) Duplicate link
2) Duplicate link
3) No

Product.aspx (or whatever display page for products you are using) is used for every product in the store. You can call it multiple ways and provide a varying amount of parameters. These URLs:

http://www.site.com/product.aspx?ProductId=####&CategoryId=###,
http://www.site.com/product-name-P###.aspx,
http://www.site.com/product-name-P###C###.aspx,
http://www.site.com/customURL.aspx

all do the same thing -- product.aspx is loaded and given the parameters in the URL.

In the case of a custom URL, it is not a 301 redirect. A 301 redirect would mean your browser requested "http://www.site.com/customURL.aspx" and was told to go to "http://www.site.com/product.aspx?ProductId=####&CategoryId=###" instead. I don't think this is what AC does. With AC, I'm pretty sure it checks the URL against its list of redirects and loads the appropriate page, without your browser having to do anything else.

To see it in action, start changing the URL and see what loads. You'll notice that as long as the ProductId parameter is there (whether as ?ProductId=### or P###.aspx) AC will find the right product.

1. If I update the name of a product the link changes. Is this some type of a redirect, or just duplicate links.

If you update the name yes it changes the product URL. Its not duplicate URL its new URL and It means previous URL will no longer work. It means if you previous link is indexed on Google now its broken.

2. If I add a custom url, is that a redirect or again just a duplicate link

Custom URL is a new URL not duplicate. Custom URL is different then auto generated URL. Auto generated URL have to follow a specific pattern but custom URL could be anything of your choice as long as its unique.

3. If I add a fixed redirect using the seo feature is that a '301' type redirect?

This helps you to deal with SEO issues. For example you can use this to keep your old indexed links in above two cases. For example you had product in your catalog for a year or so and it was indexed in Google. Now for some reason you have to change the product name or you are going to make use of custom URL. It means your previously indexed URL is going to break. You can add redirect in system where you can tell system map my old URL to my new URL. In this case when ever some one will request the old URL which is no longer available system will serve the product through new URL by doing 301 redirect. This will help you preserve your rankings.

If you update the name yes it changes the product URL. Its not duplicate URL its new URL and It means previous URL will no longer work. It means if you previous link is indexed on Google now its broken.

Its not broken as far as I understand and the new link is duplicate link technically.
A product with a name say "My Test Product" has a url like http://www.mystore.com/My-Test-Product-P21.aspx
If you change the name to "My New Test Product", the new URL will be http://www.mystore.com/My-New-Test-Product-P21.aspx

The old URL http://www.mystore.com/My-Test-Product-P21.aspx is still valid. It will take you to the same page as the new URL http://www.mystore.com/My-New-Test-Product-P21.aspx

There is a canonical URL for the page that Google will index. The canonical in the case above will change, however, Google will still be able to reach the same product page and re-index the page with new canonical URL without any problem. If you do not want any URL to change at all, use custom URL for the product. In the custom URL just specify the old URL http://www.mystore.com/My-Test-Product-P21.aspx