What is the difference between "LA" and "CD" in methylphenidate?

No difference. What a pharmacist relayed to me was that each different formulation/supplier lab of the generic methylphenidate >Extended Release< type uses a different name (initials) for their own formula of "extended release" mechanism. He didn't say more but I have to assume it is for identifying each formulation from a specific lab since they are each (supposedly) different. The initials, LA = L(ong) A(cting) for Ritalin, or ER or CD mean the same "action". Each signifies their own formula so they can be referred to specifically instead of using the same initials for them all.

METADATE CD (methylphenidate hydrochloride extended-release capsules) is a central nervous system (CNS) stimulant. The extended-release capsules comprise both immediate-release (IR) and extended-release (ER) beads such that 30% of the dose is provided by the IR component and 70% of the dose is provided by the ER component.
Ritalin LA® (methylphenidate hydrochloride) extended-release capsules is an extended-release formulation of methylphenidate with a bi-modal release profile. Ritalin LA® (methylphenidate hydrochloride extended-release capsules) uses the proprietary SODAS® (Spheroidal Oral Drug Absorption System) technology. Each bead-filled Ritalin LA (methylphenidate hydrochloride extended-release capsules) capsule contains half the dose as immediate-release beads and half as enteric-coated, delayed-release beads, thus providing an immediate release of methylphenidate and a second delayed release of methylphenidate.