From Pega Help

Use of Triggeractivities is recommended and avoids a warning condition reported when you save this rule. You can type in the name of an existing activity of another Activity Type, but the activity must conform to these restrictions:

• The activity cannot itself commit database transactions, because a triggered activity runs during database commits. - is this to avoid locking, infinite loop and override issues?
• Preconditions and transitions in the activity cannot use when condition rules and cannot call functions.

Can anybody help me in understanding the restrictions here.