Custom Improvements allow you to apply situational/conditional bonuses to your character. If you have an Adept Power that can be turned on or off, such as Attribute Boost, or a friend casts a Spell on you that applies a bonus or penalty to you, Custom Improvements allow you to apply those temporary bonuses. For example, Mr. Adept has STR 5 and the Attribute Boost (STR) Power at Rating 3. Since this power isn't always in effect, Chummer doesn't apply the bonus on its own (it only creates Improvements for bonuses that are always in effect). We'll need to create a Custom Improvement for his new Power so we can turn it on and off as needed.
Select the Improvements tab, then click the Add Improvement button.
Since the Improvement will be affecting his STR, select Attribute from the Improvement Type list. When you select a type, help information is shown in the window which describes what the Improvement will affect and how the values will affect it. Next we need to select which Attribute this will affect. Click the Select Value button, then choose STR from the list. Attribute Boost increases the Attribute's value by 3, so enter 3 in the Value field. Finally, give the Custom Improvement a name that's meaningful to you. When everything looks good, click OK.
When a Custom Improvement is created it is automatically activated and its bonuses applied to the character. You can toggle the bonus on or off by clicking the Active checkbox.
With the new Custom Improvement active, we can see that Mr. Adept's STR is now effectively 8! When we're done using the power, go back to the Improvements tab and turn off the bonus by clearing the Active checkbox. This will remove the Custom Improvement's bonus from the character until we turn it on again.
If you no longer need the Custom Improvement, for example a Spell effect from an enemy has finally worn off, you can remove it entirely by clicking the Delete button.