What does "SP-E", "16/5" and "20/6" mean in the flyback transformer product titles

I can help with a bit of it. There are traditional E shaped ferrite core sizes, known as (say) EF 16 and EF 20. The ferrite itself (not the entire transformer) has approximate dimensions of 16 mm width, and 5 mm thick for the EF 16. For the EF 20 cores, those two dimensions are approx 20mm and 6mm. So I think they are referring to that the transformers are using ferrites of those dimensions.

I've no idea what SP-E means though. 

SP refers to Bourns Smart Power Transformers Series

https://www.bourns.com/docs/rohs-cofc/sp_transformer_coc.pdf?sfvrsn=1b4e58f6_3

From digikey site:

SmartPower Safety 3 W to 22 W Flyback Transformers

Bourns' transformers feature an E16/5 or E20/6 power-ferrite core shape with a high clearance and creepage distance

Safety 3 W to 22 W Flyback Transformers Bourns' SmartPower safety 3 W to 22 W flyback transformers can transfer up to 22 W and are optimized for a working frequency of 132 kHz.

The transformer offers two outputs with reinforced insulation for individual operation, parallel, or in-series connection, and includes one bias winding for controller feedback.

The input voltage range varies from 85 VAC to 265 VAC with an operating temperature range of -25°C to +125°C. The SmartPower safety transformers are based on a standard E16/5 or E20/6 power-ferrite-core shape with a high clearance and high creepage distance coil construction designed for various silicon controller families.

https://www.digikey.com/en/product-highlight/b/bourns/wide-band-emc-filter-chokes

https://www.digikey.com/en/product-highlight/b/bourns/smartpower-safety-flyback-transformers

Aha. Thanks Shabaz.

Excellent. Thanks javagoza 

Actually, that's another thing. I was curious as to where the 85 to 265 VAC came from as it is not stated anywhere in the datasheets.

It is on the dasheet i.e.

Ah, so it is. Thanks. Still, I find it strange that it's not listed under the Electrical Specifications section.

The voltage range isn't for the transformer, it's for the SMPS that it is intended to be designed into. Manufacturers of such supplies seem to have come to a consensus that a universal input power supply should work with something like that range going in (it may form part of some of the standards, but I'm not familiar enough with them to say offhand): it encompasses the range of domestic supplies all the way up from Japan with its low 100V to the countries with a domestic supply of 240V, which includes the UK where I am.

On the transformer symbol, I think it's just indicating how each winding was intended to be used. It's not really part of the spec. Perhaps a little misleading, but a designer of SMPSs would understand what it's indicating.