What does 'model no 5TLD3ZA/A' mean on my iPhone?

Yes they do, as does the 5 character Axxxx number that you see when you tap on the Model Number.

On the other hand, the serial number can be any random series of letters and numbers.

From Google: In an iPhone model number, the first letter typically indicates the source or origin of the device, with "M" representing a new retail unit, "F" for a refurbished unit, "N" for a replacement unit, and "P" for a personalized (engraved) unit; essentially, the first letter tells you whether the phone is brand new, refurbished, or a replacement. 

The 5 character Axxxx number tells you what specific region or regions in the world that the phone was designed for.

adar224 wrote:

No, I was told it was new and brand new. When I review the information in the settings section, there is no warning about changing parts. and battery percentage is 100%

Then whoever you bought the phone from (and we know it wasn't bought from Apple) was less than truthful. But the fact it was refurbished by an Apple Authorized Refurbisher likely means it has authentic Apple parts in the phone. If there were unauthorized parts in the phone, you'd see a notification.

And no, I haven't had any parts replaced. But I ONLY buy brand new iPhone's from Apple directly and if I ever need to have a repair, it would ONLY be done by Apple directly.

No, I was told it was new and brand new.

Well, the bad news is that you bought a used phone, so you might want to speak to the folks who sold it to you. At minimum, you deserve a 15-20% refund.

The good news is that your used phone has a new battery.

What does it say if you tap Settings > General > About and look for warranty coverage? You want to see that the phone is covered by AppleCare. If it says "Coverage Expired", there is no warranty on the phone.

That it's been refurbished by someone other than Apple in an Apple-approved program; your particular one is a black 128GB model originally sold in Singapore.

(258946)

Applewatchobsessed1210 wrote:

This number is used to tell apart iPhones usually. I’m not really sure what it’s for either but I can tell you that it doesn’t matter much,

WHAT? The number matters a LOT! Sorry, but that's just nonsense. And you say you're not really sure what it's for, but we've explained what the number means for this user.

A model which starts with the number 5, means it was refurbished not by Apple, but by an Apple Authorized Refurbisher.

They usually do and most definitely does for the phone you own. Since you purchased a used iPhone, were you not told it was refurbished before you bought it?

This number is used to tell apart iPhones usually. I’m not really sure what it’s for either but I can tell you that it doesn’t matter much,

This number is used to tell apart iPhones usually. I’m not really sure what it’s for either but I can tell you that it doesn’t matter much,

It matters a lot.