What does it mean when a reference genome annotation has "plus" add to the name?

Question

[Yotam ConstantiniBiostar]

Recently i encountered a manuscript (tabula muris) that used "mm10plus" genome as a reference, but i couldn't find any explanation to the plus

https://github.com/czbiohub/tabula-muris/issues/233#

Is it a regular practice?

Answer 1

[GenoMax]

Answer is in the thread you linked:

The -plus part is the spiked ERCC RNA references

Is it a regular practice?

Not really. See: What is ERCC spike-in really saying about my sequencing run?