Looking on the official Catastro website - www1.sedecatastro.gob.es - at my property, which is out in the campo, I see nice blue lines and some red lines.
The red lines are I believe, construction and/or a subplot(s).
The blue line then is the official limit/border of my land?

If there are red subplots marked within the blue border lines - can there be someone who owns a subplot that is in my blue border that I don't know about? Or is anything within the blue border officially mine?

I can only tell you from a Catastro printout I have of my property (non campo) entitled

Consulta descriptiva y grafica de datos catastrales de bien Inmueble

it has an explanation of what the lines/markings mean in the left-hand corner - mine has no red lines

Thick lines = limite de manzana (originally one big plot - now several houses)
Thinner lines = limite de parcela

Thanks anyway. Still useful info.

Maybe ask in the town hall?

Probably will have to. Was trying to avoid the hassle of going. Not the most helpful where I am.

There's a blue info icon at the bottom right of the screen, if you click it a new page opens in Spanish with a legend, there's an option in the top-right to Google Translate the page to English.

There's a blue line which says Hidrografía but that's water.

Purple - street block limits
Black - plot limits
Red - building/subplot limits

Yes, thanks. That's the thing I'm wondering about, the red lines. I probably will go to the town hall which is about a 25 min drive away and ask them if the sub plots are mine or have been claimed by someone else. The concern is at some time the sub plots have become sub plots and I've no idea why at the moment. There's no buildings on them and my buildings are lined in red but identifiable.

I have a campo house and all the lines are red. No blue ones at all.

DLC - I've took a screenshot of part of the whole thing but to illustrate that I've looked at your suggestion regarding the info icon but it does seem the blue line in my case are just outlining the land I have. I can't see at all any water marking in blue but that isn't surprising as there isn't any there anyway.
The white area is a track, the red lines are the end of the house and maybe the house plot but there is also a red line far right that just disappears over my border when I turns to black which is the next plot of land not shown on the screen shot.

Anyway, it isn't of a concern too much but I am curious.

I can't work out what a thick blue line is supposed to mean.

But you can get a certificate of everything you own here. You need NIE + Support number, digital certificate, or clave móvil.

"You need NIE + Support number, digital certificate, or clave móvil." - I'm grateful for the info and at one point in the past I may well have seen it but, and I'm not deliberately being difficult, but - "Aye, there's the rub" said William Shakespeare, out of those listed, I just have an NIE number. Just the number. No cards, no nothing as I've never needed them. That's a whole different discussion.
I think best, as suggested, I just go the the office and baffle them with my language skills to get the advice and info I need.
Same with trying to get a Note Simple/written Catastro online. Not enough cards, numbers, electronic this and that. My fault I know.

Just bear in mind that Catastro information is frequently at odds with the Land Registry. Also the details on your Escritura may be different as well!

Nice how they make life easy.

It is worth getting digital access to be able to see all sorts of info including tax, health, DGT, catastro etc. Some townhalls can arrange to give you this digital access.

more here

https://clave.gob.es/en/registro/como-puedo-registrarme

I ought to have a closer look at that. Thanks.