Under PDGA rules, your lie must be marked before your next throw, and a mini marker disc is the accepted method. Playing from an unmarked lie, or marking with the thrown disc when the rules require otherwise, is a common source of avoidable strokes in sanctioned play.
Why it matters beyond the rulebook. Marking accurately protects your position. A mini placed carelessly can cost several inches, and several inches on an approach is the difference between a putt inside the circle and one outside it.
Practical note. Carry two. Minis are among the easiest items to lose, and finding yourself without one mid-round in a sanctioned event is an unnecessary problem.
Specification
PDGA-approved dimensions for marking your lie
Prodigy bar stamp
Available in multiple colours
At $1.99 this is a straightforward addition to any order. Free shipping on orders over $60.