Not all adventurers settle for off-the-shelf armor. Master smiths can modify armor, tailoring it to its user’s needs.
While these modifications add to the price (and often the weight) of the modified armor, customized gear can prove its worth on the battlefield. These modifications are added to mundane armor after creation at the listed cost. Modifying magical armor increases the listed cost of modifications by 50%. A suit of armor can bear only one modification.
In addition to altering the price and weight of the modified armor, each armor modification has a drawback.
In cases where the drawback reduces the wearer’s speed, the slow and steady dwarf racial trait allows the wearer to ignore the speed reduction. The Armor Adept feat allows a skilled wearer to ignore penalties imposed by armor-modification drawbacks.
https://www.d20pfsrd.com/equipment/armor/armor-modifications/