Modification Factor K12

[HotPotato69]

I'm struggling to define the use of the modification factor K12 in timber design and what it accounts for.

Can you guys explain it or point me in the right direction?

I know that its based on the modulus of elasticity and slenderness factor but I have no idea why exactly its needed or what it accounts for,

 

[Toby43]

Buckling...