What is the difference between isSaleable() and isAvailable()?

Question

I'm working on the display of the stock availability on the (individual) product page of my Magento theme, and there's something I don't completely understand about this.

I see two methods being used in the templates to check whether a product is available for sale:

    Mage_Catalog_Model_Product::isAvailable()
    Mage_Catalog_Model_Product::isSaleable()

My own findings:
I see that isSalable() (which in turn is called by isSaleable()) calls isAvailable() but also dispatches two events (catalog_product_is_salable_before and catalog_product_is_salable_after).

On the frontend, I've noticed that in Magento's base template isAvailable() is used to decide whether to display the product as "in stock" or "out of stock"; isSaleable() is used to decide something like whether to show an "Add to Cart" button.

On the backend, I've noticed that when the stock quantity becomes zero and backorders are not allowed, the stock availability of a product goes to "out of stock". When the stock quantity becomes zero and backorders are allowed, the stock availability of the product remains unchanged.

Question:
The properties "stock availability" and "stock quantity" are obviously linked with each other and the mentioned PHP methods. I would like to know:

• what the semantic difference between the PHP methods isAvailable() and isSaleable() is and why I would use one over the other;

• what it is I appear not yet to know about their relation with these properties and Magento's behavior.

Thank you.

EDIT:
I've tried every relevant combination of stock quantity (-1,0,1), stock availability (in/out of), and backorders (on/off) for a product and this is the result:

St.Qu  BckOrd  St.Av  isSalable()  isSaleable()  isAvailable()
   -1       0      0            0             0              0
   -1       0      1          N/A           N/A            N/A
   -1       1      0            0             0              0
   -1       1      1            1             1              1
    0       0      0            0             0              0
    0       0      1          N/A           N/A            N/A
    0       1      0            0             0              0
    0       1      1            1             1              1
    1       0      0            0             0              0
    1       0      1            1             1              1
    1       1      0            0             0              0
    1       1      1            1             1              1

Just for the sake of completeness:

St.Av 0  = out of stock
St.Av 1  = in stock
BckOrd 0 = no backorders allowed
BckOrd 1 = backorders are allowed

It is the stock availability switch in Magento that controls the return value of all of the PHP methods, but when backorders are off and stock quantity drops below 1, the stock availability will automatically be reset to 'out of stock' (hence the N/A rows).

Answer 1

If I'm not mistaken, the difference in these checks has to do with reservations of products for placed orders. When a customer adds products to an order, these products will still be in your stock, so they are still available. Though, they aren't saleable, because they have already been ordered by another customer.

So the semantic difference is:

• saleable means: in stock and not ordered by another customer yet
• available means: in stock but ordered by another customer, so available, but not saleable.

You could try to validate this by placing an order for a product. And doing the same check as you already did. This should cause a difference between the amount of available products and the amount of saleable products.

Edit: More info here:

https://blog.magestore.com/magento-multi-source-inventory-msi/#a3