aarch64 asm syntax & braces in SVE instructions

I've been searching everywhere but maybe not asking the right question.

In asm output with SVE instructions, sometimes I see things like

ld1d z3.d, ....

and sometimes

ld1d {z3.d}, ....

What's the difference?  Other use of {...} is for a list of source data registers, but here there's only one register and is the target.