What is the difference between" i.county|trend " and " i.county##c.trend "?

[Alko Lyu]

In the process of learning the code, I found this way of writing: "i.county|trend" (county is the code of the county, and trend is the time trend variable) , does this represent County specific time trend ?
I know that County specific time trend can be set in this way: "i.county##c.trend", do both Settings represent County specific Time Trends?

[Andrew Musau]

"i.county##c.trend"

is equivalent to

i.county c.trend  i.county#c.trend

See

help fvvarlist

I guess that you are looking at very old syntax predating factor variables where

xi: regress depvar i.county|trend

would be equivalent to

regress depvar trend i.county#c.trend

[Alko Lyu]

Hi, Andrew:

Thanks for your prompt response. Your answer helps me a lot！