Some manufacturers state that soft goods such as ropes and dogbones can be damaged by markers so it is best practice not to mark them with sharpies or other generic markers. If you do wish to mark them, there are special markers designed for the purpose. For example, see this one by Beal.
It's possible that it could damage the soft gear,
Tests done by the UIAA Safety Commission and some rope manufacturers have shown that marking
ropes with liquids such as those provided by felt-tipped pens can damage them; even with those
markers, sold specifically for marking ropes. The test results have shown a decrease of up to 50% of the