µC/OS-III handles event posting from interrupts using two different methods: Direct and Deferred Post. The method used in the application is selected by changing the value of OS_CFG_ISR_POST_DEFERRED_EN in os_cfg.h. When set to 0, µC/OS-III uses the Direct Post Method and when set to 1, µC/OS-III uses the Deferred Post Method.
As far as application code and ISRs are concerned, these two methods are completely transparent. It is not necessary to change anything except the configuration value OS_CFG_ISR_POST_DEFERRED_EN to switch between the two methods. Of course, changing the configuration constant will require recompiling the product and µC/OS-III.
Before explaining why to use one versus the other, let us review their differences.