A-Flex is a Philips-Respironics brand-specific term for CPAP Exhalation pressure relief.
According to the Philips-Respironics website: "A-Flex is designed to work with our clinically proven auto CPAP algorithm to improve comfort even more by matching pressure delivery to the patient’s entire breathing cycle. Like C-Flex, A-Flex provides flow-based pressure relief at the beginning of exhalation. Like Bi-Flex, A-Flex softens the pressure transition from inhalation to exhalation to provide additional comfort in an auto-CPAP mode. The pressure profile mirrors the patient’s breathing, and with the auto algorithm intelligence, it will respond to the patient’s therapeutic need throughout the sleep session." 
A-Flex (C-Flex+) Comfort Feature The device consists of a special comfort feature called A-Flex if Auto-CPAP therapy is enabled (or C-Flex+ if CPAP therapy is enabled). When A-Flex (C-Flex+) is enabled, it enhances patient comfort in three ways:
1) by smoothing the transition between the end of inhalation and the beginning of exhalation,
2) by providing significant pressure relief during the beginning of exhalation, and
3) by reaching an end exhalation pressure of no more than 2 cm H2O below the high point of inspiration.
In the diagram, the dashed line represents CPAP pressure in comparison to the bold line representing A-Flex (C-Flex+). A-Flex (C-Flex+) levels of 1, 2, or 3 progressively reflect increased pressure relief during the beginning of exhalation.
With A-Flex (C-Flex+), the level of pressure relief at the beginning of exhalation is determined by the A-Flex (C-Flex+) setting and the amount of patient flow in any one breath. Note: The patient also has access to this setting, if A-Flex (C-Flex+) is enabled.
Note: A-Flex (C-Flex+) transitions from no A-Flex (C-Flex+) at 4.0 cm H2O to full A-Flex (C-Flex+) at 6 cm H2O. A-Flex (C-Flex+) is top limited at 20.0 cm H2O pressure.