so confused about the box scheme in unsteady flow

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so confused about the box scheme in unsteady flow

Mehdi
Hello everyone
i found in a lot of articles that HEC-RAS (1D) uses the four-point implicit scheme, also known as the box scheme, for unsteady flow .. and as i found too that the box scheme means θ = 0.5 (implicit weighting factor), while in HEC-RAS the value of θ by default is 1, and it could be reduced to 0.6 for more accuarcy, i'm so confused !!
how it could be " box scheme " and θ is between 0.6 and 1, and not 0.5 !!
and Also if θ = 1 it means it's a full implicit scheme ..
i'm sorry i didn't get the idea or i don't  really understand the issue !
Thanks.