Please look at the these and the following link

David Kenny (http://davidakenny.net/cm/identify.htm ) lists the following conditions for identification in terms of the number of indicators per latent variable. Note condition 3 for single-indicator latent variables.

"Condition B: Sufficient Number of Indicators per Construct
For each of the constructs in the model, at least one of the following three conditions must hold:

1. The construct has at least three indicators whose errors are uncorrelated with each other.
2. The construct has at least two indicators whose errors are uncorrelated and either

a. both the indicators of the construct correlate with a third indicator of another construct but neither of the two indicators' errors is correlated with the error of that third indicator, or b. the two indicators' loadings are set equal to each other.

3. The construct has one indicator which meets either of the following
conditions:
a. its error variance is fixed to zero or some other a priori value (e.g., the quantity one minus the reliability times the indicator's variance) or
b. there is a variable that can serve as an instrumental variable (see Rule C under Identification of the Structural Model) in the structural model and the error in the indicator is not correlated with that instrumental variable."

他说的限制指的是你要设定，你选中那个error，右键，属性，里面有variance，设置为1就可以啊.这个只有一个的像是类似那种path model

There are assumptions using SEM.  Look at the following link
http://psychology.concordia.ca/fac/kline/library/k13a.pdf

Hope it helps