This is what I found in my research, great article on LMR-400 and it's variants.
"LMR UltraFlex is a close runner up in terms of popularity. This cable was created to offer easier flexing abilities for repeated or periodic flexures. On the whole, it is an easier cable to work with when routing the cable.
The LMR Ultraflex cable achieves better flexing abilities by employing a stranded centre conductor and a rubber (TPE) outer jacket.
Better Flexing Abilities come at a cost of 15% higher attenuation than the LMR-400 however!
It is worth noting, should you choose the LMR-UF as your preferred option, you can only use ‘TC’ style connectors with the cable – as ‘EZ’ connectors are not recommended for use with the LMR-400-UF stranded centre conductor."
Article:
https://www.mcgillmicrowave.com/lmr-400-variant/