Theoretical background
According to Eurocode 2nd order effects (EN 199211 §5.1.4 (1)P) and imperfections must be taken into account.
 For the 2nd order effects (EN 199211 §5.8.5 (1)), you may do
 either a full 2nd order calculation
 either use one the two simplified methodes:
 method based on nominal stiffness (see EN 199211 §5.8.7): not implemented in Diamonds
 method based on nominal curvature (see EN 199211 §5.8.8)
In this method the 2nd order effects are taking into account by increasing the 1st order bending moment M_{Ed} with a nominal second order moment equal to e_{2} x N_{Ed}.M_{Ed.reinforcement} = M_{Ed} + e_{2} x N_{Ed}
e_{2} [mm] is function of the effective length (syn. buckling length).
 The imperfections can be represented by an inclination θ_{i}. The inclination θ_{i} may be taken into account (EN 199211 §5.2 (7)):
 either by translating the inclination θ_{i} to equivalent horizontal forces (= ‘Global imperfections’ in Diamonds)
 either by translating the inclination θ_{i} to an accidendental eccentricity e_{i}
In this method the imperfections are taking into account by increasing the 1st order bending moment M_{Ed} with an additonal moment equal to e_{i} x N_{Ed}.M_{Ed.reinforcement} = M_{Ed} + e_{i} x N_{Ed}
The default value for e_{i} is often set to 20mm in acccordance to EN 199211 §6.1(4).
So if you would use the simplyfied method for both the 2nd order effects and the imperfections, you will end up with the following bending moment:
M_{Ed.reinforcement} = M_{Ed} + e_{2} x N_{Ed} + e_{i} x N_{Ed} = M_{Ed} + N_{Ed} x (e_{2} + e_{i})
In Diamonds
Depending on the selected options during the elastic analysis , you will have to (un)check some options in the Buckling parameters window. An overview:
Case 1: assume you do a 1st order calculation [Recommended option]
⇒  Then:
So, in the window you must:

Case 2: assume you do a 1st order calc. + global imperfections
⇒  Then:
So, in the window you must:

Case 3: assume you do a 2nd order calc. + global imperfections
⇒  Then:
So, in the window you must:

Case 4: assume you do a 2nd order calc.
⇒  Then:
So, in the window you must:
