so i've been accessing all my Linux machines from both Windows, Macs, and Android devices using IPv6, and up till now i've just used the link-local addresses to connect. I've recently learned about the ULA address ranges. Is there a functional difference between connecting to/from local LAN devices via one way or the other?

Unique Local Addresses can be routed, while link-local addresses can not (as the IPv6 network in which they belong exists on all interfaces simultaneously).

Addresses in the Unique Local Address range must be manually assigned, while link-local addresses are generated automatically.

A Unique Local Address (if assigned) has no special significance, while a link-local address must exist on each IPv6-enabled interface as these addresses play a crucial role in providing core IPv6 services (such as Neighbor Discovery).

i see. so ULA addresses can be added with just "ip -6 addr add" correct? and i can subnet them with prefixes just as global addresses?

Exactly.

The "unique" part of a ULA is actually meant to be globally unique. Since there's no central authority for allocating prefixes (as that's sort of the whole point of locally assigned addresses), RFC 4193 specifies a pseudo-random algorithm to be used for generating the lower 40 bits of the 48 bit prefix (the upper 8 bits being fc00::/7 plus the "L" bit, giving fd00::/8). The result will be a "reasonably unique" local 48-bit ID.

The fc00::/8 block (fc00::/7 with the "L" bit unset) is currently unused and a mechanism for allocating addresses from this block "may be defined later" (again quoting RFC 4193). In other words, don't use addresses from that block (yet).

great info/RFC link. thanks.